Planning to install solid core wet wall panels/tiles like these because installing shower over bath:

They are 38cm L x 65cm W each and tongue and grooved. However the house I purchased has some rows of tiles over bath like thus (and a couple of tiles behind the wash basin too):

This makes it tricky and I don't know what to do about the step change if I were to panel over the tiles. The wall behind is just drywall (maybe greenboard) over studs.
- Should I pack the wall(s) with say ply? How do I do this - just normal ply and screw them to studs? As you can see the existing tiles have a trim on top which is not flush with the tiles - it juts out by about 1 or 2 mm so to make it flush/plumb should i plaster a thin layer over the tiles?
- Should I bite the bullet and take off the tiles? Likely going to damage the drywall behind. Do I need to rip the drywall, get big section(s), cut them to size and screw them to studs? Do I need to level the walls or the tiles will be OK even if not absolutely level (eg if the adhesive provides a "give" or a cushion).
- Do I need to take paint off or just score it randomly and attach the panels (or even stick it over paint)?
Edit: For comments asking if the wet wall panels are the same as traditional tiles, no they are not. The one in the pic is this one for example. The core is MDF or some PVC composite etc.