can i suggest Tagscanner?
TagScanner (Windows, free)
or
mp3tag (Windows/Mac, free)
In mp3tag:* define a new Action (menu:
Actions\Actions)
* it will be available in
'Actions' menu afterwards.
One-shot operations are possible from the menu:
*
'Actions\Actions (quick)', or
*
'Convert\Tag - tag'-> Field: GENRE,
-> Format string: $regexp(%genre%,'Hard Rock','Nu Metal',1)
(1 for case-insensitive replacements, an option not available in $replace)
In TagScanner:* select
'EDIT' tab (upper bar)
* define a new Transformation (upper right corner) and tick its check box
* use
'Save' button to apply all the ticked transformations