FYI: I just now had a need to do some Genre cleanups myself.
I used Bex's
Tagging Inconsistencies script to find low usage genres.
Navigate to Library->Files to Edit
Expand Tagging Inconsistencies node
Expand Genres node
Expand Few Tracks node
Bob's your uncle.
Now you have a review list and can edit track data/genres completely within MM.
Sort of off topic, but, it is best to look for a native or integrated (e.g. the Tagging Inconsistencies script) solution to an update problem.
If the list is a bit long, I just pick an arbritary point to stop cleaning up for a while and go back to the "node" later.
Cogito cogito ergo cogito sum. (Ambrose Bierce)
I drink therefore I am. (Monty Python)
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Scripts: Add/Remove Playstat|Advanced Duplicate Find & Fix|Album Art Tagger|Backup|Batch Art Finder|Calculate Cover Size|Case&Leading Zero Fixer|DB_Audit|DB_Clean|DB_Schema|Event Logger|Genre Finder|Lyricator|Magic Nodes|MM2VLC|Monkey Rok|MusicIP Tagger|PUID Generator|RegExp Find & Replace|Right Click for Scripts|Scriptreloader|SQL Viewer|Stats(Filtered)|Tagging Inconsistencies