diff --git a/mod/forum/lib.php b/mod/forum/lib.php
index ccef8dcba77c5..8994e713475c1 100644
--- a/mod/forum/lib.php
+++ b/mod/forum/lib.php
@@ -214,6 +214,14 @@ function forum_update_instance($forum, $mform) {
 
     $DB->update_record('forum', $forum);
 
+    $modcontext = get_context_instance(CONTEXT_MODULE, $forum->coursemodule);
+    if (($forum->forcesubscribe == FORUM_INITIALSUBSCRIBE) && ($oldforum->forcesubscribe <> $forum->forcesubscribe)) {
+        $users = forum_get_potential_subscribers($modcontext, 0, 'u.id, u.email', '');
+        foreach ($users as $user) {
+            forum_subscribe($user->id, $forum->id);
+        }
+    }
+
     forum_grade_item_update($forum);
 
     return true;
diff --git a/mod/forum/subscribe.php b/mod/forum/subscribe.php
index a7ea07bcf95cc..c10d099469306 100644
--- a/mod/forum/subscribe.php
+++ b/mod/forum/subscribe.php
@@ -111,6 +111,12 @@
             redirect($returnto, get_string("everyoneisnowsubscribed", "forum"), 1);
             break;
         case FORUM_INITIALSUBSCRIBE : // 2
+            if ($forum->forcesubscribe <> FORUM_INITIALSUBSCRIBE) {
+                $users = forum_get_potential_subscribers($context, 0, 'u.id, u.email', '');
+                foreach ($users as $user) {
+                    forum_subscribe($user->id, $forum->id);
+                }
+            }
             forum_forcesubscribe($forum->id, FORUM_INITIALSUBSCRIBE);
             redirect($returnto, get_string("everyoneisnowsubscribed", "forum"), 1);
             break;