Maybe I don't understand your question, but I don't see how refusing an upgrade would be against your room type guarantee. The room type guarantee is based on your reservation, not the upgrade. Since the room type guarantee doesn't cover upgrades, I would expect that you could certainly refuse it (if you had wanted two doubles, for example).

I would guess that when you checked in and accepted the upgrade, your room was blocked to someone else that indicated a preference for that room type. That would explain the "bumping someone out of their room" comment, which was quite unprofessional I must add.