I don't think the housings will be the same for Halogen vs Xenon. If you look at other cars with them, they are different.
For example, E46 headlight shape is the same across all models, but the Xenons on BMW and just about every other car maker have auto-leveling mechanisms, lenses that keep the light on the road and out of people's eyes. The housing that supports this is different. In the case of my car, even the Bi-xenon and the xenon housings are different.
Like you said, an upgrade may be possible, I'm just betting it will cost some serious coin though.
I don't mind waiting for the 2nd or 3rd model year anyways to see what changes they make.
BTW why doesn't the STI have fogs?