Alternatively, you can use another MC library for fixing the tags. You create a new library with Library Manager and import to that library first.
If you want to use an external tagger I'd recommend Mp3tag (
www.mp3tag.de/en). It can correctly handle the user and proprietary tags that were created with old MC and MJ versions (prior MC13). Those tags are stored in ID3V2.3 COMM frames with descriptive headers. Many other programs support only a single COMM frame (= the regular Comment field) and may destroy any additional COMM frames. Unless Tag&Rename has changed recently it can't handle those.