I've got a Larvitar that I found in the Dream World, with its hidden ability Sand Veil. I'd been planning on using a Tyranitar in my team, so I thought I'd check it's IVs out. I was surprised to find it has 31 in both Attack and Defence, as well as a total between 121 and 150, so I knew it was a good Pokemon.
However, I wanted Tyranitar to have Sand Stream, which isn't a hidden ability. I don't know how they work, but I'm assuming that the Tyranitar I evolve from this Larvitar will also have a hidden ability - is this correct, and if so, is there no way I can change this?