Can I extend an existing tile tub surround to create a shower surround?
To do it well (so you won't regret it later) you will need to strip back to the studs and start with cement-board. More mess now, less trouble later.

In replacing a ~1964 tub which had a tile surround up to "shower height" I found that the whole thing was built on drywall. In that case I was having to rip tile to replace the rusted-out sheet-steel tub, but wanted to minimize offense to the rest of the tiled bathroom - I was able to salvage enough tile to trim around a fiberglass tub surround and maintain the overall look of the bathroom. I cement-boarded behind the surround, in the event that someone might ever want to tile the whole thing. Finding matching tile was not happening, and not all the tiles came off intact.