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.
i do it a on a regular basis when clients want to update the doors but dont want to change out the jambs.. sometimes we cant change the jambs without the finished floor being effected
i woiuld check the jamb for level and square first before swapping the door. you can get a pretty good indication how much work will be involved .. you might only have to shave a 1/8" off the new door sides or the bottom.. or you might have to scribe both teh top and sides for a irregular jamb..
AS Pelz and Woodwork says.... Yes.... All the time.....probably 100 or more.
My area in Colorado was peppered with hollow core flat (no panel) crap doors. (As was my home)
I usually replaced with a filled core (not solid wood... but there is no difference except cost) often from the BB when they have a sale,
The filled core slab is a lot quieter/heavier/and you can get different panel configurations... and still not expensive.
As Pelz and wood say, just jig up and do it. I'd do 5 or 10 doors at a time... carry them up to the shop.... use them as a template for the new slab.
Hardware mortises can be cut easily with a Jig (maybe $30 with router bit) and hardware lock holes have a very easy jig also that is probably 10.(My porter cable jig uses a couple of small nails to position it... I found it easier/faster/accurate to pre-drill a small guide hole.Just and idea)
Painting is probably the most time consuming part... I hang them on a chain clothes line and spray them with an airless.
It's easy, and I think a vast improvement over the crap builders doors.
"And as Pelzer says"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." When you have your router set and tools layed out, you can pound them out.
My only problem was getting rid of the old slabs.... (I'm sure not on our garbage man's x-mas card list)
Good luck