If the tile and grout are in fine shape...nothing loose, no crack in the tile of grout...and you can handle the added gain in floor elevation, you can tile over it. Two things you want to do are clean/degrease the floor and scuff up the surface of the tile a bit.

For the scuffing, I use 36 or 40 grit on a belt sander. It's not the only way, there are no hard and fast rules. I recommend a grit 80 or lower, and a palm or random orbit sander can work to. You're not looking to take the glaze off...you just want to get rid of the gloss by sctratching up the surface of the tile to give the thinset some tooth to grab a hold of.

Thinset. I recommend using bagged thinset that is unmodified. Sometimes referred to as "dryset". Then modify it on your own with a liquid latex additive. Modifying it yourself gives a better product.