Welcome, Mike.
I would not, under any circumstances, rebuild a tile shower re-using the old receptor. You, of course, may do as you like.
How old was the shower when you tore it out?
Do you know for certain that there is a pre-slope under the waterproof liner?
No, you can't do as you suggest. To effectively change to a Kerdi Shower System you must install the Kerdi drain, which will require that you remove the existing clamping drain. There is an adapter allowing you to install a bonding flange drain on top of your existing clamping drain, but I would not recommend that in your application, either.
If you want to change to a direct bonded waterproofing membrane shower system (Kerdi is one such), you want to start by removing everything you have down to your subfloor.
My opinion; worth price charged.