diff --git a/lib/classes/task/refresh_mod_calendar_events_task.php b/lib/classes/task/refresh_mod_calendar_events_task.php
index ca5a63ee25d..34dd6d28f30 100644
--- a/lib/classes/task/refresh_mod_calendar_events_task.php
+++ b/lib/classes/task/refresh_mod_calendar_events_task.php
@@ -59,10 +59,9 @@ class refresh_mod_calendar_events_task extends adhoc_task {
                 continue;
             }
             // Check if the plugin implements *_refresh_events() and call it when it does.
-            $refresheventsfunction = $plugin->name . '_refresh_events';
-            if (function_exists($refresheventsfunction)) {
-                mtrace('Calling ' . $refresheventsfunction);
-                call_user_func($refresheventsfunction);
+            if (component_callback_exists('mod_' . $plugin->name, 'refresh_events')) {
+                mtrace('Refreshing events for ' . $plugin->name);
+                component_callback('mod_' . $plugin->name, 'refresh_events');
             }
         }
     }
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 8e08d74eae1..21b4ebe58ab 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2566,19 +2566,6 @@ function xmldb_main_upgrade($oldversion) {
             $dbman->add_field($table, $field);
         }
 
-        // Create adhoc task for upgrading of existing calendar events.
-        $record = new \stdClass();
-        $record->classname = "\\core\\task\\refresh_mod_calendar_events_task";
-        $record->component = 'core';
-        // Next run time based from nextruntime computation in \core\task\manager::queue_adhoc_task().
-        $nextruntime = time() - 1;
-        $record->nextruntime = $nextruntime;
-        $DB->insert_record('task_adhoc', $record);
-
-        // This same task is queued again in a later step, but if we already queue it here
-        // then there is no need to queue it again. We use this flag in the second step.
-        $refresheventsadhocadded = true;
-
         // Main savepoint reached.
         upgrade_main_savepoint(true, 2017030700.00);
     }
@@ -2671,21 +2658,6 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2017040402.00);
     }
 
-    if ($oldversion < 2017040403.00) {
-        // Create adhoc task for upgrading of existing calendar events.
-        $record = new \stdClass();
-        $record->classname = "\\core\\task\\refresh_mod_calendar_events_task";
-        $record->component = 'core';
-
-        // Next run time based from nextruntime computation in \core\task\manager::queue_adhoc_task().
-        $nextruntime = time() - 1;
-        $record->nextruntime = $nextruntime;
-        $DB->insert_record('task_adhoc', $record);
-
-        // Main savepoint reached.
-        upgrade_main_savepoint(true, 2017040403.00);
-    }
-
     if ($oldversion < 2017040700.01) {
 
         // Define table oauth2_issuer to be created.
@@ -2838,25 +2810,6 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2017041801.00);
     }
 
-    if ($oldversion < 2017042600.01) {
-        // If the previous step didn't execute and queue the task.
-        if (!isset($refresheventsadhocadded)) {
-            // Create adhoc task for upgrading of existing calendar events.
-            $record = new \stdClass();
-            $record->classname = "\\core\\task\\refresh_mod_calendar_events_task";
-            $record->component = 'core';
-
-            // Next run time based from nextruntime computation in \core\task\manager::queue_adhoc_task().
-            $nextruntime = time() - 1;
-            $record->nextruntime = $nextruntime;
-            $DB->insert_record('task_adhoc', $record);
-
-        }
-
-        // Main savepoint reached.
-        upgrade_main_savepoint(true, 2017042600.01);
-    }
-
     if ($oldversion < 2017050500.01) {
         // Get the list of parent event IDs.
         $sql = "SELECT DISTINCT repeatid
@@ -2894,5 +2847,20 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2017050500.02);
     }
 
+    if ($oldversion < 2017050900.01) {
+        // Create adhoc task for upgrading of existing calendar events.
+        $record = new \stdClass();
+        $record->classname = '\core\task\refresh_mod_calendar_events_task';
+        $record->component = 'core';
+
+        // Next run time based from nextruntime computation in \core\task\manager::queue_adhoc_task().
+        $nextruntime = time() - 1;
+        $record->nextruntime = $nextruntime;
+        $DB->insert_record('task_adhoc', $record);
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2017050900.01);
+    }
+
     return true;
 }
diff --git a/mod/assign/db/upgrade.php b/mod/assign/db/upgrade.php
index 5afc8889b68..bc8f955fdff 100644
--- a/mod/assign/db/upgrade.php
+++ b/mod/assign/db/upgrade.php
@@ -253,35 +253,6 @@ function xmldb_assign_upgrade($oldversion) {
         upgrade_mod_savepoint(true, 2017021500, 'assign');
     }
 
-    if ($oldversion < 2017031000) {
-        // Set priority of assign user overrides.
-        $params = [
-            'modulename' => 'assign',
-            'courseid' => 0,
-            'groupid' => 0,
-            'repeatid' => 0
-        ];
-        // CALENDAR_EVENT_USER_OVERRIDE_PRIORITY has a value of 9999999.
-        $DB->set_field('event', 'priority', 9999999, $params);
-
-        // Set priority for group overrides for existing assign events.
-        $where = 'groupid IS NOT NULL';
-        $assignoverridesrs = $DB->get_recordset_select('assign_overrides', $where, null, '', 'id, assignid, groupid, sortorder');
-        foreach ($assignoverridesrs as $record) {
-            $params = [
-                'modulename' => 'assign',
-                'instance' => $record->assignid,
-                'groupid' => $record->groupid,
-                'repeatid' => 0
-            ];
-            $DB->set_field('event', 'priority', $record->sortorder, $params);
-        }
-        $assignoverridesrs->close();
-
-        // Assign savepoint reached.
-        upgrade_mod_savepoint(true, 2017031000, 'assign');
-    }
-
     if ($oldversion < 2017031300) {
         // Add a 'gradingduedate' field to the 'assign' table.
         $table = new xmldb_table('assign');
@@ -318,18 +289,6 @@ function xmldb_assign_upgrade($oldversion) {
         // Execute DB update for assign instances.
         $DB->execute($sql, $params);
 
-        // Create adhoc task for upgrading of existing mod_assign calendar events.
-        $task = new \stdClass();
-        $task->classname = "\\core\\task\\refresh_mod_calendar_events_task";
-        $task->component = 'core';
-
-        // Next run time based from nextruntime computation in \core\task\manager::queue_adhoc_task().
-        $nextruntime = time() - 1;
-        $task->nextruntime = $nextruntime;
-        // Indicate to the adhoc task that only the assignment module will be refreshed.
-        $task->customdata = json_encode(['plugins' => ['assign']]);
-        $DB->insert_record('task_adhoc', $task);
-
         // Assign savepoint reached.
         upgrade_mod_savepoint(true, 2017042800, 'assign');
     }
diff --git a/mod/assign/lib.php b/mod/assign/lib.php
index 30c472c9ff0..860f5ad3624 100644
--- a/mod/assign/lib.php
+++ b/mod/assign/lib.php
@@ -231,14 +231,17 @@ function assign_update_events($assign, $override = null) {
             $conds['groupid'] = $override->groupid;
         }
     }
-    $oldevents = $DB->get_records('event', $conds);
+    $oldevents = $DB->get_records('event', $conds, 'id ASC');
 
     // Now make a to-do list of all that needs to be updated.
     if (empty($override)) {
-        // We are updating the primary settings for the assign, so we need to add all the overrides.
-        $overrides = $DB->get_records('assign_overrides', array('assignid' => $assigninstance->id));
-        // As well as the original assign (empty override).
-        $overrides[] = new stdClass();
+        // We are updating the primary settings for the assignment, so we need to add all the overrides.
+        $overrides = $DB->get_records('assign_overrides', array('assignid' => $assigninstance->id), 'id ASC');
+        // It is necessary to add an empty stdClass to the beginning of the array as the $oldevents
+        // list contains the original (non-override) event for the module. If this is not included
+        // the logic below will end up updating the wrong row when we try to reconcile this $overrides
+        // list against the $oldevents list.
+        array_unshift($overrides, new stdClass());
     } else {
         // Just do the one override.
         $overrides = array($override);
@@ -272,6 +275,7 @@ function assign_update_events($assign, $override = null) {
         $event->timesort    = $event->timestart + $event->timeduration;
         $event->visible     = instance_is_visible('assign', $assigninstance);
         $event->eventtype   = ASSIGN_EVENT_TYPE_DUE;
+        $event->priority    = null;
 
         // Determine the event name and priority.
         if ($groupid) {
diff --git a/mod/lesson/lib.php b/mod/lesson/lib.php
index 7a6ab2500e8..e838f5d3be3 100644
--- a/mod/lesson/lib.php
+++ b/mod/lesson/lib.php
@@ -121,14 +121,17 @@ function lesson_update_events($lesson, $override = null) {
             $conds['groupid'] = $override->groupid;
         }
     }
-    $oldevents = $DB->get_records('event', $conds);
+    $oldevents = $DB->get_records('event', $conds, 'id ASC');
 
     // Now make a to-do list of all that needs to be updated.
     if (empty($override)) {
         // We are updating the primary settings for the lesson, so we need to add all the overrides.
-        $overrides = $DB->get_records('lesson_overrides', array('lessonid' => $lesson->id));
-        // As well as the original lesson (empty override).
-        $overrides[] = new stdClass();
+        $overrides = $DB->get_records('lesson_overrides', array('lessonid' => $lesson->id), 'id ASC');
+        // It is necessary to add an empty stdClass to the beginning of the array as the $oldevents
+        // list contains the original (non-override) event for the module. If this is not included
+        // the logic below will end up updating the wrong row when we try to reconcile this $overrides
+        // list against the $oldevents list.
+        array_unshift($overrides, new stdClass());
     } else {
         // Just do the one override.
         $overrides = array($override);
@@ -167,6 +170,7 @@ function lesson_update_events($lesson, $override = null) {
         $event->timesort    = $available;
         $event->visible     = instance_is_visible('lesson', $lesson);
         $event->eventtype   = LESSON_EVENT_TYPE_OPEN;
+        $event->priority    = null;
 
         // Determine the event name and priority.
         if ($groupid) {
diff --git a/mod/quiz/lib.php b/mod/quiz/lib.php
index 7a9300a2d99..de6d1de0def 100644
--- a/mod/quiz/lib.php
+++ b/mod/quiz/lib.php
@@ -1205,14 +1205,17 @@ function quiz_update_events($quiz, $override = null) {
             $conds['groupid'] = $override->groupid;
         }
     }
-    $oldevents = $DB->get_records('event', $conds);
+    $oldevents = $DB->get_records('event', $conds, 'id ASC');
 
     // Now make a to-do list of all that needs to be updated.
     if (empty($override)) {
-        // We are updating the primary settings for the lesson, so we need to add all the overrides.
-        $overrides = $DB->get_records('quiz_overrides', array('quiz' => $quiz->id));
-        // As well as the original quiz (empty override).
-        $overrides[] = new stdClass();
+        // We are updating the primary settings for the quiz, so we need to add all the overrides.
+        $overrides = $DB->get_records('quiz_overrides', array('quiz' => $quiz->id), 'id ASC');
+        // It is necessary to add an empty stdClass to the beginning of the array as the $oldevents
+        // list contains the original (non-override) event for the module. If this is not included
+        // the logic below will end up updating the wrong row when we try to reconcile this $overrides
+        // list against the $oldevents list.
+        array_unshift($overrides, new stdClass());
     } else {
         // Just do the one override.
         $overrides = array($override);
@@ -1251,6 +1254,7 @@ function quiz_update_events($quiz, $override = null) {
         $event->timesort    = $timeopen;
         $event->visible     = instance_is_visible('quiz', $quiz);
         $event->eventtype   = QUIZ_EVENT_TYPE_OPEN;
+        $event->priority    = null;
 
         // Determine the event name and priority.
         if ($groupid) {
diff --git a/version.php b/version.php
index 0b7efa7f9c7..4f31d564ec5 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2017050900.00;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2017050900.01;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.