if it were me i'd take it all down, plasterboard and all then reboard. it will actually save heaps of time in the long run. trying to remove the tiles without damaging the pb is very painstaking and, frankly, virtually impossible. quicker, easier and not too much more expensive to replace the plasterboard. just a bit more messy is all.
You could of course if you have enough thickness to work with e.g. skirtings & frames tile straight over after cleaning the old tiles with methalated spirit or the like. As existing tiles will be stuck fast, hence problems getting them off!
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