Assuming the bathroom wall is sheet rock, this is really not too difficult. Sheet rock is very easy to patch, so just cut a panel out from stud to stud and from the floor to about 12" high. The hole obviously needs to expose the existing supply line as well as the space the change needs to go and be large enough to allow access to work. Do the necessary plumbing inside the wall, bringing the extension over to where you want it to be. Sometimes it's easier to patch the sheet rock if you sister 2x4 blocks on the studs to act as a nailer for the patch. I find2-1/2" or 3" deck screws and a power driver superior to nails when mounting these blocks. Tape and mud the joints and your done.