Preserved url's path. Only the domain will change now.

src/manifest.json

@@ -15,6 +15,7 @@
     "128": "assets/images/icon128.png"
   },
   "permissions": [
+    "tabs",
     "storage",
     "webRequest",
     "webRequestBlocking",

src/pages/background/background.js

@@ -688,12 +688,19 @@ browser.runtime.onInstalled.addListener((details) => {
 });
 
 
-function openPage() {
-  if (oldDomain != '') {
-    browser.tabs.update({
-      url: oldDomain
-    });
-  }
+function changeInstance() {
+  browser.tabs.query({
+    active: true,
+    lastFocusedWindow: true
+  }, function (tabs) {
+    var tabUrl = new URL(tabs[0].url);
 
+    if (oldDomain != '') {
+      browser.tabs.update({
+        url: tabUrl.href.replace(`${tabUrl.protocol}//${tabUrl.host}/`, oldDomain)
+      });
+    }
+  });
 }
-browser.pageAction.onClicked.addListener(openPage);
+
+browser.pageAction.onClicked.addListener(changeInstance);