/diy/ - Do It Yourself

I am trying to remodel a shower in my house that's on a slab. It looks like the cut a hole out in the concrete for the shower area, about 3-4 inches below floor level, and then filled it with concrete to make a shower pan. I don't want to learn how to pitch concrete correctly, so I was just going to buy one of those expensive shower pan kits from kerdi or similar company. My question is, do I need to take the old pan out completely and get down to the original foundation? Can I reuse the old one even though it's covered in thinset already? Can I put concrete on top of the old one to get it level, then add the foam pan?