The only rule is that you must be able to
play it as well as
sing it in the new key. Choose any key which suits your voice for that particular song. If, for example, you find that the keys D and Db work well for singing that song, then choose the guitar friendly key of D, rather than Db.
Every chord
type in the new key will be exactly the same in the new key. So Bbmin7 will still be some form of minor 7 in the new key, but whether it's Amin7 or B min7 or whatever depends on which new key you're transposing to.
Using a capo, however, gets around that problem. All the keys become equally easy to play in that case - (but still not equally easy to sing until someone invents a voice capo.
