It sounds like the "equipment change" was an attempt at a polite way of taking away whatever error or favor had been give the OP. I think there are two basic questions in this thread, similar to the horrible experience at the SFO Int'l First lounge that RichardinSF experienced. First, "Who has the right and the responsibility to enforce the rules?" and more importantly, "When should a 'bank error in your favor' be rescinded?"

I would argue that in this case the GA has the right to check that no one is getting more than they paid for. However, does she have the responsibility to ensure that no one gets more than they paid for? I thought that the responsibility of ensuring that policies and rules are followed falls to the audit process. The GA may also not have the context for why something is the way it is. And once the GA makes a decision to reverse whatever goodwill someone else at UA had offered, it is hard to rectify after the trip has been taken.

And more importantly, I have to agree with the OP. There is no sense of entitlement here. In this case, the GA was perfectly within their rights to take this action. But I think taking away the 'bank error in your favor' costs UA much more in the long run in terms of goodwill than the cost of letting the occasional mistake go overlooked.