Hmm, i not agree with you Peter, if you open a hi_carname.tex file, select the 256*256 and save it as carname.exe, you will not have a file using the same format as other carname.tex.
Proof is that if you open you just create file, you will see it is still a 512*512 tex file and not a 256*256 like carname.tex files use.
The correct way to do it is to open the hi-carname.tex file, select the 256*256 mipmap size and choose "extract seleted file" to save the texture in tga.
Then you have to open the old carname.tex file you wish to update, select the 256*256 mipmap and choose "replace "selected, using the tga you just saved.
Next you need to click on downsample to update each mipmap and then you can save the tex to replace original carname.tex file with your new texture.