You can use DXTBMP by Martin Wright to open up and change formats
But that won't be your problem.....or at least it's unlikely to be that simple.
It sounds like certain textures are in the wrong place and the model is not finding them.
This can be a common problem when FS9 models are used in FSX especially if shared folder structure is used.
A lot of this sort of thing can occur in the VC.
You might get a better answer if you could be more specific into what textures are missing from which version
It could be a number of things.