The plan is for the BUKs to be available in Arena Commander and for you to be able to change them using the holotable, although we can't guarantee that this will happen the day the Vanguard becomes flight ready. (It's likely, for instance, that the base Vanguard might be hangar ready before the variants, in which case variant owners would get a loaner ship to fly.) The limiting reagent here is that there are some additional weapons and technologies (see next question) that are being developed for the variants independently of the ships themselves (the same is true of the UI/process for switching them via the holotable.)