I dunno if 0.2 is anything to go by, as well as Nomura's statement "you can trigger those transformations by fulfilling the right conditions" I don't think we're getting menus for the keyblade transformations. We will probably fulfill the transformations requirements and then above the menu a situation command appears to activate the transformation. Just like how Aqua can trigger two different styles (wayfinder and spellweaver) but can't select them from a menu she can just choose to activate them when the right conditions were fulfilled.
Base Sora will probably have access to all transformations of an available keyblade making him the most versatile, able to switch between them like you can jump from spellweaver to wayfinder for example. While going into a drive form will auto transform your keyblade into the appropriate transformation and lock it into that form until the drive form is exited. So base Sora can trigger the shield or the chariot, Guard form can only use the shield, and Power form can only use the chariot.
At least that's how I believe is the most likely way for it to work. It also fits what we saw with the star keyblade as it to had two transformations. One into a set of dual guns which fires lots of little shots everywhere and keeps Sora purely ranged. While the other is a single large gun which Sora swings around like lance, catching enemies then releasing a single powerful shot on the captured enemies at point blank range. They also had some kind reticle modes the dual one is like being on a machine gun turret and the single one is just one big powerful focused shot. I could realistically see how one of these is for a guard form, as being purely ranged and firing everywhere would indeed keep Sora out of harm's way. While the the other one which is melee based + single very concentrated shots is for power form.
It also could be possible to jump from one drive form to another, probably from the Guard Form to the Power Form. If we have in mind that in the first trailer of 2015. Sora goes from the Guard Form to Power Form, along with the prototy footage shown exclusitively in the KH2.5 event.