diff --git a/course/modlib.php b/course/modlib.php
index 4eb9d0656e2..5fae8ab5b98 100644
--- a/course/modlib.php
+++ b/course/modlib.php
@@ -491,12 +491,17 @@ function update_moduleinfo($cm, $moduleinfo, $course, $mform = null) {
     }
 
     $completion = new completion_info($course);
-    if ($completion->is_enabled() && !empty($moduleinfo->completionunlocked)) {
-        // Update completion settings.
-        $cm->completion                = $moduleinfo->completion;
-        $cm->completiongradeitemnumber = $moduleinfo->completiongradeitemnumber;
-        $cm->completionview            = $moduleinfo->completionview;
-        $cm->completionexpected        = $moduleinfo->completionexpected;
+    if ($completion->is_enabled()) {
+        // Completion settings that would affect users who have already completed
+        // the activity may be locked; if so, these should not be updated.
+        if (!empty($moduleinfo->completionunlocked)) {
+            $cm->completion = $moduleinfo->completion;
+            $cm->completiongradeitemnumber = $moduleinfo->completiongradeitemnumber;
+            $cm->completionview = $moduleinfo->completionview;
+        }
+        // The expected date does not affect users who have completed the activity,
+        // so it is safe to update it regardless of the lock status.
+        $cm->completionexpected = $moduleinfo->completionexpected;
     }
     if (!empty($CFG->enableavailability)) {
         // This code is used both when submitting the form, which uses a long
diff --git a/course/tests/behat/activities_edit_completion.feature b/course/tests/behat/activities_edit_completion.feature
new file mode 100644
index 00000000000..e099938ae52
--- /dev/null
+++ b/course/tests/behat/activities_edit_completion.feature
@@ -0,0 +1,57 @@
+@core @core_course
+Feature: Edit completion settings of an activity
+  In order to edit completion settings without accidentally breaking user data
+  As a teacher
+  I need to edit the activity and use the unlock button if required
+
+  Background:
+    Given I log in as "admin"
+    And I set the following administration settings values:
+      | Enable completion tracking | 1 |
+    And I log out
+    And the following "courses" exist:
+      | fullname | shortname | enablecompletion |
+      | Course 1 | C1        | 1                |
+    And the following "activities" exist:
+      | activity | course | idnumber | name     | intro | content | completion | completionview |
+      | page     | C1     | x        | TestPage | x     | x       | 2          | 1              |
+    And I log in as "admin"
+    And I follow "Course 1"
+
+  Scenario: Completion is not locked when the activity has not yet been viewed
+    Given I turn editing mode on
+    And I click on "Edit settings" "link" in the "TestPage" activity
+    When I expand all fieldsets
+    Then I should see "Completion tracking"
+    And I should not see "Completion options locked"
+
+  Scenario: Completion is locked after the activity has been viewed
+    Given I follow "TestPage"
+    When I follow "Edit settings"
+    And I expand all fieldsets
+    Then I should see "Completion options locked"
+
+  @javascript
+  Scenario: Pressing the unlock button allows the user to edit completion settings
+    Given I follow "TestPage"
+    When I follow "Edit settings"
+    And I expand all fieldsets
+    And I press "Unlock completion options"
+    Then I should see "Completion options unlocked"
+    And I set the field "Completion tracking" to "Students can manually mark the activity as completed"
+    And I press "Save and display"
+    And I follow "Edit settings"
+    And I expand all fieldsets
+    Then the field "Completion tracking" matches value "Students can manually mark the activity as completed"
+
+  @javascript
+  Scenario: Even when completion is locked, the user can still set the date
+    Given I follow "TestPage"
+    And I follow "Edit settings"
+    And I expand all fieldsets
+    When I click on "id_completionexpected_enabled" "checkbox"
+    And I set the field "id_completionexpected_year" to "2013"
+    And I press "Save and display"
+    And I follow "Edit settings"
+    And I expand all fieldsets
+    Then the field "id_completionexpected_year" matches value "2013"