That was a given from the initial release.

Typically, Flight Simulator offers full compatibility with the immediately preceding version.  In this case, FS-X, will run FS-9 models (those made to the FS9 SDKs) without (significant) change.  There may be some minor texture issues.

The basic flight dynamics are unchanged.  From a modelling standpoint, the changes are eye-candy (graphics/textures) and some more animation variables available for a modeller to tweak.

If your model flies terrible in FS9, it'll fly terrible in FS-X,  but it will look better...