There is always the possibility that there may be backwards compatibility. FSX does have it, but some modifications were needed to make them look better. That is why there was a flood of FSX updates to the then current FS9 addons (at least there was with aircraft). Over time there came the native FSX addon aircraft. It just took the developers a while to learn the new ways of FSX. It might be the same for Microsoft Flight. Only time will tell.

I had not bothered with scenery addons in FS9. I have only done that since FSX. So I can't really say that side went.

Well that's my two bobs worth.

Mark

EDIT: Microsoft's policy on the MS Flight Simulator series has always been that they are made for computers that are not yet available. This, supposedly, means that they will have a longer life. I know it took me a few years before I had a powerful enough computer to be able to get a good performance from FS9. It was/is the same for FSX.