diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index 9bfd066bf1..c6d70e536f 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -241,7 +241,8 @@ function GenerateClassFromInfo(info, Base, behaviors) {
     // Prevent element from initializing properties when it's upgrade disabled.
     /** @override */
     _initializeProperties() {
-      if (!this.hasAttribute(DISABLED_ATTR)) {
+      // Enable disable-upgrade use when `legacyOptimizations` setting is on.
+      if (legacyOptimizations && !this.hasAttribute(DISABLED_ATTR)) {
         super._initializeProperties();
       } else {
         this.__isUpgradeDisabled = true;
diff --git a/test/unit/disable-upgrade.html b/test/unit/disable-upgrade.html
index bf430f648f..47bca562ad 100644
--- a/test/unit/disable-upgrade.html
+++ b/test/unit/disable-upgrade.html
@@ -77,6 +77,10 @@ <h2 id="element">[[prop]]</h2>
 import { DisableUpgradeMixin } from '../../lib/mixins/disable-upgrade-mixin.js';
 import { Class as PolymerClass } from '../../lib/legacy/class.js';
 import { Polymer } from '../../lib/legacy/polymer-fn.js';
+import {setLegacyOptimizations} from '../../lib/utils/settings.js';
+
+// This setting is for all tests...
+setLegacyOptimizations(true);
 
 Polymer({
   is: 'x-disabled-legacy',