Thanks for your feedback, Shawn.

I've had little time for Übercart in the past few days, so I'm late on the response. But today I installed the new beta, and I can be more complete in my reply.

1. Still as before. I'll check whether I'm doing something wrong, or I'll file a specific bug report.
2. This totally works now, thanks! (So now basically my issue is solved.)
3. I guess removing a role while adding one isn't always necessary, except if one wants to make sure that users with the upgraded role can't buy any upgrades until the role expires. So: basic role -> product is available, role upgradable; upgraded role, basic role removed -> product is not available, role extensions not available. (A solution for control freaks, I realize that.)
4. Got it. I hadn't looked carefully enough.

Once again, thanks a lot.