Well (I'm only guessing here, by the way), I would think we'd have to fill out another form, so besides that form maybe mentioning/requiring your Live ID, you'd probably be filling in other details for it from scratch. Why I say this is that MS will have to know your bank account and tax information - all of which you didn't supply when signing up for a Live ID.
The most that will happen is they may fill out part of your form using your Live ID information (which many do for an easier experience in signing up for something else, if you've signed up with them for something else already), but you'd most likely be able to edit that information still.
