It is hard to say how easy or hard it will be to retile, or if it really should be just retiled verses tearout completely and start over. But, once the tile is off (and you may find the only way to do that would tear up what's beneath so much it costs about the same to replace the whole thing), changing the valve is the easy thing. A full new shower could easily cost $6k or more in NYC. Depending on the age of the thing, the shower pan may be on borrowed time. It's really hard to tell.