Replacing existing floor tiles over wet underfloor heating

All depends of which sort of screeding has been done on top of the heating pipe. If it is a soft screeding once you remove the old tiles you have to redo the screeding. If the screeding was hard screeding or a self leveling compound was used just remove the old tiles and you can retile straight after. When removing the tiles do not dig straight down but side ways so you will not hurt the pipe works.

Tricky one to answer really. I'm assuming it's the plastic style piping you mean. If so then hopefully it's been well screeded and as Roberto has said just be careful when removing them. Hopefully whoever tiled the floor didn't just put the tile adhesive straight over the piping without screeding (things like this do happen 🙄). If that was the case you would have problems. Maybe whoever you purchased the house from will know what was done. If not, just begin carefully if going ahead and you will see what's what as you go. It's one of those jobs where unless someone knows what's under there, then you won't know until you start. Good luck.