The existing tiles must be soundly bonded to the walls. Obviously, no matter how well fixed the new tiles are, if the old tiles de-bond from the wall and fall off, then so will the new ones.
Tap old tiles to see if they sound hollow, as this will tell you if the tile is or is not fully bonded to the wall.
If the old tiles and new tiles are fairly average and not large heavy formats it shouldn't be a problem.
Remember plasterboards\plaster have a recommended weight limit per sqm, plasterboards around 33kg per sm, plaster is around 20kg sqm if I remember correctly.
Remember to score the old tiles first to give the adhesive a good grip.
Bal Grip might be a good choice to eliminate slipping, but I'd dare say standard Bluestar would suffice, unless if it's in a wetroom, then maybe go for White or green star.
If the existing tiles are nice and flat then pricing should be the same as standard tiling onto fresh good plaster really.
Oh and if the new tiles are roughly the same size as the old tiles, then it might be worth trying to setout a little different to the old tiles, "depending on how your cuts will fall though" so each of your new tiles covers 2 of the old tiles "staggerd" if you know what I mean.
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And remember to clean and de-grease the existing tiles completely.
Sugar soap is a good cleaner, then let the walls dry completly.
