I have had access to the MSFS2024 Developer Alpha for a while and have had the opportunity to test my MSFS2020 sceneries in the new sim. They all work relatively well in MSFS2024, although there are some issues.
If you have purchased my sceneries for MSFS2020 from either simMarket or the SimNord store and want to use them as soon as MSFS2024 is released, you can simply copy the packages from the 2020-Community folder to the 2024-Community folder. If you have purchased my sceneries in the Marketplace, according to what Microsoft has stated, you should have access to them in the new sim automatically.
When a stable MSFS2024 SDK is released, I will update my 2020 sceneries so that they are fully 2024 compatible. I expect it will be free of charge for existing customers, but if, contrary to expectations, it turns out to be a lot of work to make my sceneries fully 2024 compatible, I may ask customers to pay a small upgrade fee.
Regarding the issues I've observed so far: There is an issue with 2020 custom aerial imagery (orthophoto scenery) not showing up in MSFS2024 Developer Alpha, there are issues with vegetation, trees, bushes etc. looking different in the new sim, reflections in windows are not as expected, and there are a few other minor issues.
But, as I said, my 2020 sceneries work relatively well in MSFS2024 Developer Alpha, and existing customers should be able to use them in MSFS2024 when it releases tomorrow.
Happy flying!