Went through this on my last house, on a shower installed in the '80s. On some of them you can just replace the cartridge, which has the seals. I believe some brands are cross-compaible which opens up options for new fixtures. Accessing what's behind the wall is not hard. Usually some small covers, set screws and maybe some caulk need to be removed to get the handles and escutcheon plate off. Then you can hopefully ID what's there. With that info you can see what your options and needs are.
Agree, if you actually do need to sweat a whole new valve in place that can get involved. Is there access from the other side of the wall? Drywall is easier to patch than tile.