All the FSX textures I found were 2048x2048 DXT5 format. For those I opened them with dxtbmp, sent to my editor (ms paint), resized to 1024x1024, saved, imported back into dxtbmp, did the same with the alpha, then saved in DXT3 format.
Actually that's not quite accurate. I changed the alphas to pure white, except where the spinner is. Some FSX repaints have alphas that don't look right to me in FS9, I think it's because of differences in the way textures are rendered and the bump mapping and such. If I knew what I was doing I might be able to make them work, but I can do without the reflections, makes the plane look a little more lived in.
I'm sure there are other members that can explain the proccess better than I can.