Update all my mp3's genres from online DB?

Ok so I used the search, and I ddin't find what I was looking for, but I may just suck at searching.

I have about 20k mp3s that are well-tagged insofar as their titles, artists, etc. are concerned, but their genres are all over the place. At some point in time, some app I used actually replaced a large volume of my genres with numbers for some strange reason.

Well, I blanked those out so I wouldn't have to look at that, but that leaves me genreless, and I would like to fix that. I figured that Mp3tag might be able to help me with the easy wins and get all the generes freedb/amazon could give me from recognized songs.

I can't figure out how to actually do that, and trying with single songs almost always brings up another menu saying "freedb thinks it might be one of these songs" which it never is.

An help would be appreciated!

Ok, this is my one bump, hopefully someone can help!

Maybe this can help:
http://forums.musicbrainz.org/viewtopic.php?id=1795
You must "unset" the tagging of the tags you do not want to change, because as default MusicBrainz Picard overwrite the old tags.
Make always a backup first and check if it all works as you want.