It is possible but not a good idea, its a lazy mans way of doing it and not very reliable. The trouble with taking the old ones off is they will also take the plaster off with them, this is not generally a big problem if the wall is going to be tiled again.
If the tiles are soundly stuck to the wall, you should have no problem. I'd wash them down first before tiling. I've done several tile on tile jobs & have had no problems.