Well, if it isn't Transformers, then what else does USH have that would be worth "cancelling a trip over" because it is coming to USO on the SS44 footprint? From reading descriptions of House of Horrors, there is no way people cancel trips over that. Transformers, sure, especially since some people have already shown how the ride footprint can fit into SS44's area. My only guess would be Transformers.
-heads to USH website to see the list of their attractions.