How big a job is swapping my own furnace really?

Did this exact swap a couple years back on my own place, same situation with a newer coil staying put. You'll almost certainly need to build a transition. Furnace heights vary between models and brands, and even if the footprint lines up, the supply air opening off the top rarely does. I bought a sheet of galvanized and bent my own transition box, but any sheet metal shop will bend one to your measurements for cheap if you'd rather not mess with it. Seal the seams with foil tape and the coil hanger straps ussually reach fine, though I added a couple self-tappers into the transition to take the weight off the lines. Don't try to move the coil itself, the refrigerant lines don't have the slack and kinking one means an evacuation and recharge you don't want to pay for.

On the gas side, the stuff that trips people up is usually the little stuff. Put a sediment trap on the vertical drop before the furnace connection, most manufacturers require it and inspectors always look for it. Use a new flex connector, never reuse the old one. And check the manifold pressure with a manometer after it's running, don't just trust the regulator to be set right out of the box. Also verify the new unit matches your gas supply, natural vs propane matters if anything got swapped around over the years.

Two more things worth checking before you button it up. First, confirm the new furnace's blower speed and static setup works with your existing coil and ducts, a mismatched setup can cause the coil to freeze up in summer even if heat works fine. Second, if the new unit is a higher efficiency condensing type, plan the PVC venting and condensate drain now, not after it's in the hole. That's the part that catches people, the old flue pipe comes out and suddenly you're routing two plastic pipes and a drain line you didn't budget wall space for. Measure the whole path before the old unit comes out.

Agree with the poster above about planning the venting route before the old unit comes out, learned that the hard way here. Swapped to a condensing model two winters ago and totally forgot the old chimney flue was a single hole in the masonry. Ended up patching it with a cover plate and running the PVC out the side wall, which meant another hole through the siding and a longer run than I wanted. Also the condensate wouldn't gravity feed to my floor drain because the furnace sat higher than I expected, so I had to pick up a small pump after the fact. Walk the whole path with a tape measure first.

Swapped mine out last spring and the surprise for me was the width, not the height. New cabinet was a couple inches narrower, so there was a gap at the return plenum that the old unit had been covering. A strip of sheet metal and some foil tape closed it up, but I wish I'd measured both units side by side before ordering. One tool that saved me was a right angle adapter on my drill, becuase half the screws you need to pull are buried against the cabinet wall where a regular drill won't fit. Also take a phone photo of the wiring and the gas c*ck orientation before you tear anything loose, saved me a head scratch later.

Update: pulled the trigger and the new unit shows up next week. The height mismatch point was spot on, so I'm planning a sheet metal transition off the top instead of hoping the openings line up. Still stuck on venting though. If I go condensing, the exhaust needs a new exit route since the old flue is gone. Anyone deal with routing the condensate drain when the furnace sits in a tight closet?