I've done this a couple of times. It's generally not too difficult,
but it is time consuming. You will need to use an old chisel or
perhaps a flat-bladed screwdriver. Wedge it under the edge of the tile
and pry them away. Depending upon the mastic used, and how long the
tiles have been installed you may find that the tiles pry up easily, or
you may find that they crack apart and have to be nibbled away. Either
way, you should be able to save the backerboard if you're careful.