Be warned that this messed up a lot of files for me. In many cases the track name was wiped from the file, sometimes the artist, album, and track name were all wiped out. I was able to reverse the damage after a few hours with a mass tagger (luckily the filenames had enough for me to restore most tracks), but I still wouldn’t recommend this procedure.
That’s true. If you don’t have id3v1 tags, or your id3v1 tags are wrong or empty, that will indeed propogate bad data to the id3v2 tags.