Assuming no damp or water penetration issues, it is almost impossible to remove tiles from a wall without pulling lots of plaster and chunks of plasterboard off with them. You end up with hours of labour to ensure all the old adhesive is removed and the holes patched to create a smooth surface ready for re-tiling. Plus, sufficient time must be allowed to ensure the patching is 100% dry before tiling over it. Removing and replacing the the entire section of plasterboard will be quicker and ensure a a flat stable surface the new tiles won't fall off in a year or two.I have received a quote and the installer wants to take down the four walls in our bathroom (all of which have some tiling). He doesn't want to take the existing tiles off, just rip the whole wall down. Then replace with new walls, then retile.
Scoring along the top of the wall at the ceiling will prevent damage there, As @Pinoy adventure says though, the water resistant plasterboard or cement board is much better suited to this.