About the only thing you're doing is repainting your freeware aircraft.

You can't, of course, take the payware model's textures and rename them for the freeware model's textures, since the texture mappings are *probably* not the same.

You can take the payware model's textures as references with which to paint your freeware model, just as you would use photographs, etc. as references.

BUT if you're actually copying textures from the payware model to corresponding areas of your freeware model, well, as long as they're for your personal use, it's your choice, but if you plan to upload them, then that's a different issue altogether and not proper.