The are several subsystems in MSFS, such as
- User Interface
- Core flight rules and physics
- Content Manager
- World Content Streaming
- Weather
I expect based on what is being said, that #2, #3, and #5 are going to be relatively stable and probably to some point shared between MSFS 2020 and 2024. Keep in mind when MSFS 2020 was launched, #2 was actually the FSX core. WU 5 replaced that with new modern code.
Based upon what we see in the trailer, the LOD for Los Angeles was better. The environment could be interacted with (dumping water on the fire actually impacts the fire, so not just a water animation). Other than the LOD we didnāt really get to see much of city rendering etc, but I have been keeping up with Blackshark.AI and they have made considerable advances since 2019. I am expecting that some of those advances will be included in #4. The new mission/career will probably be its own new subsystem.
So basically, if the core is the same, I would expect most the addons to react to API calls the same as they do now. But with improved graphics, LOD, ability to interact with the environment, etc I can see where a developer would want to perhaps upgrade functionality. However, those would be 2024 updates, and wouldnāt reflect back to 2020 because 2020 would not have the same updates to #1 and #4. In which case, they would have to branch off a 2024 version of the product. So your 2020 version of the product will still work, but for products that have been enhanced for new graphics, features, ability to interact with the environment (like, Roland Laborie right now on his CanadAir 500 drops water which is just an animation, but if he wanted to actually imprement the new water drop that effects a fire, that would probably then be a 2024 specific version). It would have to be a new product in the marketplace because 2020 would not support the enhancements.
So will everyone charge an upgrade cost? Iām not sure. I have to believe it would be based upon how extensive the effort to upgrade was. Of if you have initials such as CS, you might want to release new versions of even your liveries. But based on comparability being backwards compatible but not forward, I expect that you will have the choice whether to upgrade or not, and if it is worth it to you.
No these are all guesses of course. But they are based upon what has been said so far, and based upon what can be seen in the trailer.