Jonx over at Got Friends offered a good explanation of 3rd party aircraft compatibility in the new sim.

Link for Discord users: Discord

I can't talk about everything, obviously (NDAs and such), but I do want to make things clear because the community seems to be rather confused:

Before we start, Legacy = MSFS 2020 and Native = MSFS 2024

- The vast majority of "Legacy" content is backwards compatible.

- If you purchased addons in the MSFS 2020 marketplace, you will be able to install them in MSFS 2024 Marketplace.

- This also includes Community versions bought from 3rd Party Websites/Installers. MSFS 2024 has a Community Folder (Just drag them in)

- When Microsoft mentions "2024 Upgrades" that means that the "Legacy" add-on has been brought to "Native" standards to take advantage of the new features. This does not mean that the aircraft is just "Working the same as MSFS 2020", that is what backwards compatibility is for.

- You can expect a "Upgrade Fee" to happen when a very complex "Legacy" aircraft undergoes a large amount of work to make it "Native"

- These new native features are all optional, the aircraft developers doesn't have to add Walkaround Preflight or Career paths if they don't want to. So depending on what features the developer decides to add will determine the amount of work required to achieve those new features in a Native format.

- But if they put in that work, they should charge accordingly for the amount of work involved. Even if they decide to be generous and make it free, there will still be work involved. If they choose to add all the new features, this could be a heavy rework, which is why all features are optional... because it depends on if the aircraft actually has a career use-case or other areas to further explore.

- Take our Double Ender for example, does that have career use-cases? Probably not. What about our Wilga? Yes, this can be used for Skydiving, Banner Towing, Crop Dusting, Glider Towing, Passenger Hauling, Medical Evac, etc... The Wilga would undergo a lot of work to get all those careers working. This is a very fluid question and developers will be able to decide if they just want to remove chocks or integrate a deeper preflight mechanic like engine maintenance. It's all up to how deep they decide to go with these new features. If the community starts calling for "Free Upgrades", then most developers will just put in a simple amount of work and let it go. Giving those developers another revenue stream for incorporating more complex features, allows them to push their aircraft's realism even deeper. They know they can spend more time on them and make them better beyond a simple update cycle. 

- That is why you may see a small Upgrade Fee on some projects... Maybe the developer put a ton of time into adding way more features. Maybe some will be free upgrades, because the developer thought the transition was easy or they decided to be generous. This is the same time old concept of freeware vs payware. The same story that people argue about today. At the end of the road, it comes down to supporting the developer if they are good. And if they ask for a minimal fee to upgrade, I'm sure you will receive enough work to make that fee worth it. If it doesn't feel worth it, then move onto supporting another developer. It's the same story that revolves over and over again. The choice is yours, the community... but pre-emptive calls for free upgrades are not healthy for developers. Each aircraft is unique and deserves different attention for this next sim.

I don't have a problem with paying upgrade fees. Look.. I can't do what these developers do. It's a skilled trade in my opinion. They should be paid for their work. 

That was a very good response by Jonx and explains a lot from the developers point of view. 

We need to understand that yes the 2020 planes will be forward compatible more than 2024 planes will be backwards compatible.