Build shower pan over old wall tile?

[johnbgoode]

Hello,
First time posting. I'm helping a friend of a friend who recently had a basement shower installed, and unfortunately the contractor who did it was completely incompetent. The floor of the shower is tile, and there is no slope to the drain (in fact if anything, there is a hump drawing water away from the drain). I'm planning to rip the old floor out and build a new pan, but my question is: do I have to remove the first row of tiles from the wall (which of course means I need to find the matching tile to reinstall), or would it be ok to build the pan over the existing tile?
Thanks!
John

 

[Dhtiling]

How are you going to waterproof the pan? I don't see any way of doing it without removing the first row, and if the workmanship was that bad, all the other rows as well.