Maybe you didn't realize that the Premier tile website in the whole country is johnbridge.com, and Mr. Bridge himself has offered his advice to you.
First, I recommend you click his link to visit the site. There is a tremendous wealth of information there. In addition,
www.tileusa.com if the website for the Tile Council of America, the industry organization. They also offer a wealth of information on line and through inexpensive publications.
Now, there is a reason why I or Mr. Bridge or any other professionals who lurk around these forums are reluctant to suggest that you could do what you are asking. A shower is a big construction project that realistically should be a committment for the next 20 years. Also, a minor failure in the watertight construction can lead to latent damage which if you have not read about the horror stories, do a search on mold or rot.
It's not that it is improper to tile over tile. You will read in the various references I have cited that under the right circumstances it is in fact doable. It's just that a shower is made up of the substructure which is equally important as the tile itself. And sometimes a 'patch' job just is not the right thing.
So, above all, read the literature. Make yourself the most educated person on your block about tile showers! Then you can proceed with confidence.