I actually don't know if they'll really let you do that... before the livestream, the theory was that vendor gear (Vanguard, Crucible, Factions) could be leveled up to Level 33, and that Raid gear would go all the way up to 34. But Bungie's commentary during the livestream almost made it sound like that ALL armor would level up to 34 when upgraded. They're gonna have to elaborate on this at some point...