Nobody knows what will work in FSX unless they text for M$.
Don't believe all the complete BS posted by speculators.
They gave us all that rubbish for FS9 then we found the fde's needed a lot of fiddling and the shiny materials and textures where all messed about with.
I'm not making anything new until i have the FSX sdk to read and understand.
The messenger can go FSDS2 or 3 so thats not the issue.
no more questions about my WIP models as they are going nowhere at the moment as my PC is dying big time and i dare not lose any saved models or files.
To be honest, there's not many differences I can see between FS2004 aircraft and FSX aircraft. The model detail will only go as far as a modeller's skill and the only new thing is bumpmapping and maybe improved reflection as far as I can see. Model quality seems as good as it can get (same as FS2004). I'm hoping then, that FSX will be backwards-compatible aircraft-wise.