Basically, you’ll lose the PMDG Boeings, the ifly Max8 and the MD11, (if you decide to delete 2020) and gain INI’s A321 and 330, and the SAAB 340 if you care (personally I think INI’s planes alone are worth the price of a basic MSFS 2024 package)
You’ll gain the walk-around function, and possibly a substantial increase in frame rates (although many people have reported that they haven’t got that improvement I have gotten a big improvement in the cockpit, generally the walkaround feature slows framerates down a bit, but it’s not a slideshow or anything).
If you have a ton of add-ons, the load time of the game will be a fraction of what it once was (used to take me 40 minutes to load 2020 now it’s 5 minutes), loading into a flight can often take longer in 2024 as you download streamed scenery and aircraft but in total getting from computer off, to on the tarmac in 2024 is much less for me. This is a double edged sword, because 2020 can be played without the internet whereas 2024, so far, does not seem to be.
Control bindings are a bit tricky, but once set up it’s more automatic, and each plane will load settings for controls that are bound to them, instead of having to change your bindings every time you switch between planes with different control settings (ie going from a three holer with three throttles to a twin with two)
As for scenery I have one by one been enabling my airport collection from the 2020 marketplace and testing for crashes, despite there being a warning that the developer may not have certified it for 2024 I have about 15 2020 airports enabled to be streamed, none have caused a crash, two of them have had grass covering the taxiways and aprons instead of pavement, and the rest have worked perfectly fine as far as I can tell. If you have purchased scenery outside of the marketplace, some will have been approved for 2024, and some may not be, so check your vendors if there are some airports you are unwilling to part with.
GSX and Beyond ATC work fine in the sim, if you use these. I have no other quality of life type add-ons so I can’t comment on other ATC systems or anything like that.
I have a 14900k CPU and a 4060ti GPU and 32mb RAM, and I almost never use 2020, but I keep it around just in case something breaks.