It would work - the only catch is if the later flight is full on the day then you won’t be able to swap! That can be the case with EDI flights. Or I guess if your UA flight is late and you miss the hour before cut off (I always do a change with more than an hour just in case!). You will also need to be careful of time to check in bags - an hour will not be long enough to get between terminals and check in bags. I normally only do this when HBO. However, to answer your question - yes that should be able to do it as you described without any need for extra payment.

regards

aks120