We bought Rock Band 3 and want to do the song transfer which will take 1.9 gigs. Can I delete the main Rock Band 2 content to free space?
If you're talking about your Rock Band 2 save file that contains your bands and your preferences, deleting them won't help because they're so tiny to begin with.
If you mean your Rock Band songs, it might help depending on how many you have. You will always be able to download them again later on without paying again, but it might be a pain as you have to do them all individually.