Re: Interior french doors
You could reuse the existing frame, however the difficulty factor jumps by a power of 10.
You can't just slap hinges on a slab and screw it to the jamb, the hinges have to be mortised into both the door and the frame, this is difficult enough for a pro, let alone a DIY'r. Then you have to run door stop molding through the center of the jamb, this is what the doors will close against.
While you may not want to disassemble your existing jamb and molding, installing a set of prehung doors will be the way to go for a precision fit and finish. You can custom order your prehung doors to be exactly the same size as the existing opening, increasing the chances of reusing your molding.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!