diff --git a/src/clos/change.lsp b/src/clos/change.lsp
index 5fbd4cd09..a7f033749 100644
--- a/src/clos/change.lsp
+++ b/src/clos/change.lsp
@@ -215,6 +215,25 @@
 
   class)
 
+(defun slot-definitions-compatible-p (old-slotds new-slotds)
+  (loop for o = (pop old-slotds)
+        for n = (pop new-slotds)
+        while (and o n)
+        do (let ((old-alloc (slot-definition-allocation o))
+                 (new-alloc (slot-definition-allocation n)))
+             (unless (and (eq old-alloc new-alloc)
+                          (eq (slot-definition-name o)
+                              (slot-definition-name n))
+                          (or (not (eq old-alloc :instance))
+                              (= (slot-definition-location o)
+                                 (slot-definition-location n))))
+               (return-from slot-definitions-compatible-p nil)))
+        finally
+           (return (and (null o)
+                        (null n)
+                        (null old-slotds)
+                        (null new-slotds)))))
+
 (defmethod make-instances-obsolete ((class class))
   (setf (class-slots class) (copy-list (class-slots class)))
   class)
diff --git a/src/clos/standard.lsp b/src/clos/standard.lsp
index f8ee2031c..8c42aeb85 100644
--- a/src/clos/standard.lsp
+++ b/src/clos/standard.lsp
@@ -303,8 +303,14 @@ because it contains a reference to the undefined class~%  ~A"
 
     (setf (class-precedence-list class) cpl)
     (let ((slots (compute-slots class)))
-      (setf (class-slots class) slots
-            (class-size class) (compute-instance-size slots)
+      ;; We don't change identity of class-slots when slot definitions
+      ;; are compatible to avoid making instances obsolete. This is
+      ;; allowed by the standard (see MAKE-INSTANCES-OBSOLETE).
+      (if (and (slot-boundp class 'slots)
+               (slot-definitions-compatible-p (class-slots class) slots))
+          (map-into (class-slots class) #'identity slots)
+          (setf (class-slots class) slots))
+      (setf (class-size class) (compute-instance-size slots)
             (class-default-initargs class) (compute-default-initargs class)
             (class-finalized-p class) t))
     ;;