The strength of drywall rests with the paper covering either side of the plaster. If the paper is removed from the drywall, you cannot glue the tiles to the surface. Unfortunately, when tiles glued to drywall are removed, often the paper comes off with it. Try removing the tiles gently and you might avoid tearing the drywall paper. If you are unsuccessful, try the following. Once the tiles have been removed, a thin layer of plaster can be added to the wall, and the tiles can be placed on the plaster. Remember that the exposed drywall should be sprayed with water before the plaster is applied. If not, the moisture from the wet plaster will be absorbed by the drywall and the plaster will not adhere properly.