Models built for FS2004 - planename.MDL - will fly in FS2004. Models built for CFS3 - planename.M3D - will fly in CFS3. The two are not interchangeable or compatible with each other.
Some aircraft are available for both sims, but that doesn't mean they're exactly the same model. In fact, the only model I can think of which is is Milton Shupe's Beech 18. Which aircraft are actually available for either sim is a result of Microsoft and subsequent other modellers' decisions.
Or is it all a gigantic conspiracy????? :isadizzy: