CFS2 to FSX? You Bet!
I've successfully converted literally hundreds of CFS2 models (espc ships) into FSX by using simple hex editing trick. As mentioned above, replacing the reference in the .mdl file from CFS2 to FS90.
Many friends have asked me "why" bother?....My response is a resounding, why not?
I've enjoyed flights in numerous older FS version aircraft that (after replacing prop textures - this requires some experimenting to find the right one, and I have compiled a collection of various prop bmp's etc to use) work just dandy. After all, there are more older models than new ones.
Just my 'two cents" worth.