@lthenderson this seems like it's up your alley.
Sorry for the delayed response, I was on spring break so missed this post until now.
We have a bunch of crappy old birch/luan doors in our house. To make matters worse we've replaced a bunch of carpeting with LVP or revealed hardwood, so the doors are too short now. This is a typical 60's housing boom development, so we don't need to get too fancy. But I think new doors would look MUCH better. So, we want to replace about 15 interior doors. And actually the front door too, but we'll probably pay someone to do that one.
Are we crazy? I don't want to rip out the trim to do prehung doors plus they're more expensive. Not real sure if we're signing up for days or torture, or we'll get the hang of it pretty quickly?
In short, tearing out 15 doors is a lot of work and you are in for many days of torture. First off, if I were to do a project like this, and I wouldn't recommend it, I would recommend using prehung doors. It will save you lots of time. Getting hinges mortised into the new door and keeping your gaps all nice looking is very tedious work and easy to get wrong. Also with openings that shift over time, one can spend hours getting a door to fit just perfectly. Using a prehung door, eliminates all this fastidious work and is much easier to get better looking results but opens up its own can of worms.
When you remove a door to install a prehung door, the trim on both sides needs to be removed and then the prehung door installed. Often this means the prehung door doesn't sit exactly in the same location, due to different shimming and so when you go to install the trim back around the door, you end up with overhangs or small gaps that now have to be filled, or adjacent pieces of trim cut longer/shorter to make things work. Depending on your sense of aesthetics, it can be as simple as some caulking and paint or it will literally cascade into lots of new baseboard and paint.
Sometimes for various reasons, wall thicknesses can vary and one way to compensate for that is to trim or build up the existing jamb to match. So when you replace this with a new prehung door, you may end up doing the same things to get the jamb widths to match the wall thicknesses.
Reinstalling hardware in old openings, hinges and lockplates, can mean having to add screws into wallowed out holes, or lock plates that are in a slightly different position meaning there isn't material for the screw and the chiseled hole is out of place by a quarter of an inch. These can be fixed with chunks of wood, chisels and glue but take time.
I have replaced doors before but generally do so as part of a room remodeling project when everything is torn up anyway. In your case, probably the easiest and cheapest fix would have been to add another layer of sheathing over the floor before laying your new flooring down to make up the height difference and you would have ended up with more solid flooring as a result. After the fact though, you don't have that option. The gaps seem obvious now because you are focused on them. I would give it a while and see if you come to ignore them. Gaps underneath doors are generally a good thing as they allow the air to circulate better in a house. If you really need noise privacy, there are different types of kick plates and door seals that you can install to take up the gap on the bottom until such time you are ready to do a general room upgrade by painting all the walls and trim anyway and at that time make the swap.