This pertains only to scenery objects, which is true. You may need to convert also the textures, as has been said. And need to recompile them so P3D can use them in the classic way (object library bgl and textures in their respective folder).
My initial response was targeted at the scenery itself, airports, vector data, satellite images. There are fundamental differences in between the simulators. If you do not have the source files I'd say it is impossible (e.g. compiled vector and satellite data is encrypted, you can't decompile it).
If you want to have an exact overview then post the question "is it possible to convert msfs 2020 scenery to p3d (prepar3d) ?" in an AI (Grok, ChatGPT, Gemini....) of your choice. It will give you a very detailed answer, which is probably too much for this thread.
It will also describe the exact route if you want to convert scenery objects.
Cheers,
Mark