When I have done it in the past I have always chiseled the tiles off the wall as carefully as possible with a broad chisel to try not to tear up the underlayment too bad. Then you can simply smooth the wall back out with drywall mud before putting the new tile up. Make sure you wear some good thick gloves. I usually end up breaking some of the tiles and I ended up with a really nasty cut last time I did it because I had forgot my gloves.
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