If your old two-handled shower valve is worn out or leaks, don't despair. You don't have to tear open the tile wall to replace it. The best way to replace it is with a conversion plate. The plate covers the old valve holes and you do the entire job through the hole you cut in the tile. Buy a plate that fits your new faucet and is wide enough to cover the old valve positions. To find a conversion plate, call a plumbing supply store or order online.
This is a challenging project. You may find loose tiles, rusted pipe and other problems.