What's on the walls? It's often hard to impossible to remove an existing pan without tearing out at least part of the walls, and then, you might as well update the whole thing.
If you decide to do it with tiled surfaces, check out
www.johnbridge.com.
You have to figure out how to tie any moisture management between the walls and the pan, too.
I think the code says the shower pan size must accommodate at a minimum, a 30" diameter circle, so one with a 28" dimension may no longer be legal.