I too tried flashing CWM over TWRP using Flashify, but it didn't work, it still said I had TWRP. However, Flashify did work to revert back to Stock Recovery.

I still have the backup file of the Stock Recovery, I can upload it somewhere if you want to, then maybe you can install CWM.
I have a D802 myself, not sure if my backup would work with your D800 though. but if you're adventurous...