Have you tried it to see if it works as-is? Maybe it doesn't need any changes.
Many FS8 3D models work just fine in FS9. The most common reason that some don't work has to do with animations on models that were made with older software, and the most common way they don't work is that their props are invisible - an issue that wouldn't affect a jet.
Less commonly, animations like flaps and landing gear might not work. If that's the case, you're out of luck, as the only "fix" would be to rebuild the model using more up to date software that's fully compatible with FS9.
If the 3D model works but the plane doesn't fly right, maybe the flight dynamics files need a tweak, since FS8 and FS9 handles flight models somewhat differently. In most cases the difference is negligible, at least to an amateur like me, but sometimes some updating is called for. You could make adjustments to the existing flight files, or adapt the flight model from an FS9 aircraft with similar performance and handling.