I can see both sides of this issue. Yes, OP ended up getting an UG pulled and then getting one replaced ahead of others on a different flight.

If I was #1 on the list and didn't clear on the other flight, I would be bummed out. But the truth is this has happen many times to me (and I'm sure others on this board).

On the other hand, I would have done exactly what the OP did. It's not his fault UA oversold. Again, I think most of us here would want to have UA honor their confirmed UG for us at any length (well, I sure would).