One of the worst ideas ever. Don't be so sure that you won't have moisture damage behind the walls and under the floor. Gut it, or leave it alone until you are ready to gut it.

The Tile Council of North America disagrees with hollysprings:

Here is a link that might be useful: TCNA

Not saying it can't be done Treb. Shouldn't. It's not a good idea. There are so many showers ''in good condition'' that when ripped out, were anything but in good condition. Shortcuts usually mean long travel.

Tiling over tile in dry areas(not bathrooms/showers) is one thing.

I agree tiling over shower tile is a bad idea.

Even if the existing tile is in good condition---and(huge and) the waterproofing(if any) under the existing tile is in good shape, adding another layer of area where trapping water is a probability is simply creating a future problem.

I had a neighbor who did that and they regret it. Not worth any future problems, in the long run it could cost you a ton of money.