A lot depends on the condition of the wall behind the tiles.
I tiled on tile when I modernised our bathroom in 1976, four years after we bought this house. The originals which were the old-fashioned 4" tiles, had been on since 1965 when the house was built and I think were were put on with cement! They were on so well.
I went over all the walls tapping with the end of my hammer handle, checking to see if any sounded loose, but found none. It's important to check as you could have even a square yard of tiles that are only held together by the grout between them as the adhesive on the backs of them isn't firmly stuck to the wall or if the tile is on plaster, that could be coming away from the wall.. If you tiled over in such a situation a lot could come off later.
I gave all the tiles a good clean and scored them up a bit with a chisel, just to help the adhesive stick, a tiring process. Then I stuck them on with waterproof adhesive. They were still secure in 2002, when I had a contractor come in to re-tile. He just took the second layer off but the first layer was still fimly stuck to the wall. These new tiles have been up for another fifteen years, with no problems.
When tiling keep a spare carton of tiles. You never know when you might need to replace some. Even the same tile can be a different shade, if purchased a year later.