anyone ever swap out interior doors without replacing the frames?
I did exactly what you're talking about just this weekend. My doors all seem to have been replaced during an 80's remodel with those ugly hollow cores. I took the door out, popped in the new door... Too wide by about .25". So I took both doors out to the garage, placed the old door on top of the old, and marked out the cuts. In my case, they were square.
Then I layed the original door on top again and used my speed square to mark the locations for the hinges and knob. Drilled the knob, routed the hinge mortices, brought it inside, hung, everything fit perfectly. I used new hardware, as previous owners hadn't believed in masking while painting, and I can't stand those cheap kwikset handles that rattle around in your hand.
You won't regret it. A solid door both looks and FEELS better. It took me maybe an hour, and my next one will be quicker.