I'm trying to recreate my band from a previous Rock Band title for online play and it won't let me register. Why not?

Unfortunately we don't actually manage the band names - those are all managed by the network that you play on (they also manage the "classy"; filters for you band names). Usually once a band name is taken, it's gone. But there are two things that you can try here. You can reach out to Consumer Services for your console (Microsoft or Sony) to see whether they can fix the band name issue for you, or, if you'd just like to get back to playing, try using a variation of your original band name. (For example putting a zero in place of the "o" in "box", or a number elsewhere in the name).