I’d like to have a function to save only the modified albums from the selection rather than save all albums, similar to the remove complete/perfect albums plugin does.
So I thought I’d start with adding something like this
if (isinstance(album, Album) and album.get_num_unmatched_files() != 0
to the “Save and rewrite header” Plugin.
What is the purpose for this plugin vs pressing / Selecting Save?
The purpose of this plugin very likely is just to clean up broken metadata blocks. Sometimes you get files with a somehow broken metadata block. Errors there could prevent you to load the file or its tags in some software. Usually Picard will just modify the existing block, which might not fix any errors in your tag.
One way to fix this is to delete the tags completely from the file and write a new one. This is what this plugin does.
In regards to your actual problem: If you want somwthing like this the “Save and rewrite” headers plugin would be a good start. I would just leave out the deletion part 