All players should be able to class swap regardless of their current class. Armor is such a pointless limitation. Who cares if a Warlock has Plate on because they class-switched from a Death Knight? It's got the wrong stats, so it's gonna be shit gear anyways. You're gonna have to go to the AH and buy BoE greens and start back from Heroic Dungeons, and carry all that extra loot around with you in your inventory if you want to play that way. Bag space alone will make playing many different classes a massive inconvenience.