Is it possible to move the waste on a bath?

The waste pipe is usually a simple push fit that anybody can do using varios angled fittings. It will be designed to accommodate them. It is more professional to remove the tiles but tiling on top of the existing ones is fine providing you make sure they are grease free and evenYou can rough up the surface with glass paper to assist adhesion. . Any tiles finishing half way up a wall though will produce a 'step' which you have to be aware of in your design.