Don't dawdle when you pick your class and you'll have plenty of time. I've changed my class up to 4 times at the start of the game (down on the d-pad after death). If you truly want to change, suicide and change. ONOZ you'd have a single solitary death added to your k/d...big whoop.
Ah I just assumed crossfire. I didn't even realize people played blitz beyond getting their achievements ;p