I have spoken with some experience tile installers on this subject,
and what they have said is if the orignal tile is down, and has no cracks, or defects, and the customer absolutely wants to change the color of tile for example, they will tile right over the existing tile.
If there are issues with cracks through the tiles, or in the grout, they will
refuse to do this, they would require to remove the old tiles, stiffen up the subfloor, and install new tile. So, I have no experience with this at all.. but I have herd that it is commonly done, with good results.
