You will have a very difficult time installing a new valve throught the access hole in the tile. Don't mean to pick on you, but this should have been thought about and accomplished BEFORE putting up tile. It will also be very difficult to secure the new valve to braces/brackets so it cannot move around.
Your only "out" here is if you have access to the other side of that wall, such as in a hallway, a bedroom wall or closet. It is a simple matter to patch drywall, and larger patches are easier than small ones. Cut a 16" square or so hole.
No matter how this is done, remember this: you will have the water main shut off, so when the fire breaks out on the drywall paper, you can't just pee on it! Have extinquishers and pails of water standing by!