diff --git a/app/assets/javascripts/discourse/app/lib/plugin-api.js b/app/assets/javascripts/discourse/app/lib/plugin-api.js
index fb42aa6d2b5..038bff8e067 100644
--- a/app/assets/javascripts/discourse/app/lib/plugin-api.js
+++ b/app/assets/javascripts/discourse/app/lib/plugin-api.js
@@ -227,25 +227,72 @@ class PluginApi {
    * ```
    **/
   modifyClass(resolverName, changes, opts) {
-    const klass = this._resolveClass(resolverName, opts);
-    if (!klass) {
+    const info = this._resolveClass(resolverName, opts);
+    if (!info) {
       return;
     }
+    const klass = info.class;
 
-    if (canModify(klass, "member", resolverName, changes)) {
+    if (canModify(info, "member", resolverName, changes)) {
       delete changes.pluginId;
 
-      if (klass.class.reopen) {
-        klass.class.reopen(changes);
+      const changeDescriptors = Object.getOwnPropertyDescriptors(changes);
+
+      const fieldDescriptorEntries = Object.entries(changeDescriptors).filter(
+        ([key, desc]) => {
+          if (typeof desc.value === "function" || desc.get || desc.set) {
+            return false;
+          }
+          if (
+            klass.prototype.mergedProperties?.includes(key) ||
+            klass.prototype.concatenatedProperties?.includes(key)
+          ) {
+            return false;
+          }
+          return true;
+        }
+      );
+
+      if (klass.reopen) {
+        // klass is an EmberObject - use builtin ember 'reopen' feature
+        klass.reopen(changes);
+
+        if (fieldDescriptorEntries.length) {
+          // reopen sets these values on the prototype, but they might be overrides for
+          // native class fields. Those need to be overridden after the original values
+          // have been set on the instance by the constructor.
+          klass.reopen({
+            init() {
+              Object.defineProperties(
+                this,
+                Object.fromEntries(fieldDescriptorEntries)
+              );
+              this._super();
+            },
+          });
+        }
       } else {
-        Object.defineProperties(
-          klass.class.prototype || klass.class,
-          Object.getOwnPropertyDescriptors(changes)
-        );
+        // Not an EmberObject. We can override functions/getters/setters on the prototype
+        // but we don't currently have a way to override fields. Log a warning to the console
+        // if there are any overrides that look like fields.
+
+        Object.defineProperties(klass.prototype, changeDescriptors);
+
+        if (fieldDescriptorEntries.length) {
+          // eslint-disable-next-line no-console
+          console.warn(
+            consolePrefix(),
+            `Attempted to modify fields in ${resolverName}, which is not an EmberObject. ` +
+              "These changes will not take effect. " +
+              `Fields: ${JSON.stringify(
+                fieldDescriptorEntries.map(([key]) => key)
+              )}`
+          );
+        }
       }
     }
 
-    return klass;
+    return info;
   }
 
   /**
diff --git a/app/assets/javascripts/discourse/tests/unit/lib/plugin-api-test.js b/app/assets/javascripts/discourse/tests/unit/lib/plugin-api-test.js
index b73dc9f4cff..ef57f9e2239 100644
--- a/app/assets/javascripts/discourse/tests/unit/lib/plugin-api-test.js
+++ b/app/assets/javascripts/discourse/tests/unit/lib/plugin-api-test.js
@@ -4,6 +4,7 @@ import discourseComputed from "discourse-common/utils/decorators";
 import { withPluginApi } from "discourse/lib/plugin-api";
 import { setupTest } from "ember-qunit";
 import { getOwner } from "discourse-common/lib/get-owner";
+import Sinon from "sinon";
 
 module("Unit | Utility | plugin-api", function (hooks) {
   setupTest(hooks);
@@ -35,6 +36,9 @@ module("Unit | Utility | plugin-api", function (hooks) {
 
   test("modifyClass works with native class Ember objects", function (assert) {
     class NativeTestThingy extends EmberObject {
+      firstField = "firstFieldValue";
+      otherField = "otherFieldValue";
+
       @discourseComputed
       prop() {
         return "howdy";
@@ -47,6 +51,8 @@ module("Unit | Utility | plugin-api", function (hooks) {
       api.modifyClass("native-test-thingy:main", {
         pluginId: "plugin-api-test",
 
+        otherField: "new otherFieldValue",
+
         @discourseComputed
         prop() {
           return `${this._super(...arguments)} partner`;
@@ -56,10 +62,15 @@ module("Unit | Utility | plugin-api", function (hooks) {
 
     const thingy = getOwner(this).lookup("native-test-thingy:main");
     assert.strictEqual(thingy.prop, "howdy partner");
+    assert.strictEqual(thingy.firstField, "firstFieldValue");
+    assert.strictEqual(thingy.otherField, "new otherFieldValue");
   });
 
   test("modifyClass works with native classes", function (assert) {
     class ClassTestThingy {
+      firstField = "firstFieldValue";
+      otherField = "otherFieldValue";
+
       get keep() {
         return "hey!";
       }
@@ -69,23 +80,42 @@ module("Unit | Utility | plugin-api", function (hooks) {
       }
     }
 
-    getOwner(this).register("class-test-thingy:main", new ClassTestThingy(), {
-      instantiate: false,
-    });
+    getOwner(this).register("class-test-thingy:main", ClassTestThingy);
+
+    const warnStub = Sinon.stub(console, "warn");
 
     withPluginApi("1.1.0", (api) => {
       api.modifyClass("class-test-thingy:main", {
         pluginId: "plugin-api-test",
 
+        otherField: "new otherFieldValue",
         get prop() {
           return "g'day";
         },
       });
     });
 
-    const thingy = getOwner(this).lookup("class-test-thingy:main");
-    assert.strictEqual(thingy.keep, "hey!");
-    assert.strictEqual(thingy.prop, "g'day");
+    assert.strictEqual(
+      warnStub.callCount,
+      1,
+      "fields warning was printed to console"
+    );
+    assert.true(warnStub.args[0][1].startsWith("Attempted to modify fields"));
+
+    const thingy = new ClassTestThingy();
+
+    assert.strictEqual(thingy.keep, "hey!", "maintains unchanged base getter");
+    assert.strictEqual(thingy.prop, "g'day", "can override getter");
+    assert.strictEqual(
+      thingy.firstField,
+      "firstFieldValue",
+      "maintains unchanged base field"
+    );
+    assert.strictEqual(
+      thingy.otherField,
+      "otherFieldValue",
+      "cannot override field"
+    );
   });
 
   skip("modifyClass works with getters", function (assert) {
@@ -95,9 +125,7 @@ module("Unit | Utility | plugin-api", function (hooks) {
       },
     });
 
-    getOwner(this).register("test-class:main", Base, {
-      instantiate: false,
-    });
+    getOwner(this).register("test-class:main", Base);
 
     // Performing this lookup triggers `factory._onLookup`. In DEBUG builds, that invokes injectedPropertyAssertion()
     // https://github.com/emberjs/ember.js/blob/36505f1b42/packages/%40ember/-internals/runtime/lib/system/core_object.js#L1144-L1163