First, you can uninstall individual packs in Sims 3. You'd be able to uninstall just Ambitions without touching anything else.
Second, because there's a whole new town involved, I really don't think Sunset Valley and Riverview will be affected. It would take a lot more programming than what's involved in an Expansion Pack to integrate the new careers into existing towns. They'd have to re-write the entire Sims 3 game to make such a major change.
Third, as I already said, if it turns out that you can play Ambitions in Riverview or Sunset Valley, the rules of the challenge forbid you from playing another family in that same game file, so whatever an architect may or may not able to do to a town will be a moot point because you won't be able to play an architect in your challenge file at all.
Granted, this is all speculation, but there's nothing else we can do right now except speculate. Until the game is launched we won't know anything for sure. No need to mess with the rules now when nothing is certain, unless Metro feels otherwise.