The most important thing is determining the condition of the existing tile. If there are ANY cracks in the grout lines or tiles, or if they are loose in any way, the problem will just telegraph up through your new application. If there are any defects then you are best off ripping it out and re-doing the substrate with cement board.
If you are 100% confident that the original install is sound then proceed. Make sure that your new floor won't end up too high. What will the transition to the next room be like with your floor 3/4" - 1" higher? What I have done in the past is to use a belt sander or rotary disk sander with an aggressive red garnet sandpaper (the second hardest substance) to really scuff up the surface and remove the shinyness. Then I paint the entire old surface with a latex bonding agent, following the instructions, then I use a standard thinset mortar with added latex bonding agent (milk). I have had good luck doing this in the past, but I can't stress enough how important the original tile's condition is.