Can you remove doors and fit new ones in existing jambs? Yes but you need to measure and make sure the new ones are the exact same height and width and thickness. You will need slabs as any pre hung there is no guarantee hinge slots and knob holes are in the right spots

Ok so say you have a standard 30x 80 inch door. Well you can use the old door as a template for hinge slots and knob holes. But still might have to do some planing or sanding as you really have no idea what was done to make the old ones fit. But say you get a decent fit. Quite often you need to remove and reset the door stops as they are set to the slight warps of the old one and the new one might not hit right.I've done a lot of doors and sometimes they go smooth and other times you spend time playing around to get a good fit.

So as long as you are replacing a standard size with the same it shouldn't be too hard. Most can be more or less plug and play but there is always at least that "one". GL