How to Install scripts?
https://www.youtube-nocookie.com/embed/D_CVBkg4HPk?start=1

Now preserves old tags.

Warning: Greasemonkey is not supported. Use either Violentmonkey or Tampermonkey.
Make sure to export a backup before use.

Adds various tags to anime/manga entries. Can delete all previous tags. Works in your anime/manga list and in Advanced Editing pop-up. Works even if tags column is hidden. Confirmation before updating and clearing tags.

If any Fail when finished you can right-click page and go to Inspect then Console and write down the id numbers then go to your backup xml and change 0 to 1 for each then Import https://myanimelist.net/import.php and last go to edit details to update them manually with the update link.