I've looked into amending the hotel but it isn't an option available now. The UOR Hotel is a late decision we have made after looking into Universal in more detail. We would definitely factor the 2 hotels into our next Orlando visit.If possible, I would amend your reservation at POR to shorten it by two nights and check in for those nights at a UOR hotel.
I don't see the point of paying for both hotels, the saving will pay for your time at Uni and probably the taxi fare too.