Currently if a file have a genre "Progressive Metal" and MBDB genres for this song is anything else, the genres will be replaced.

There is no option to merge or keep both.

I suggest a %old_genres% variable (for example) that would hold the current/old genre tag.

This allow the user to either discard them or use %genre% and other strategies.

Maybe this was already discussed but on the matrix server and chating with other users, nobody was able to help me preserve existing genres while using MB genres.