Good point about the seat material, I'm not aware if it is different.. I talk a little about the leather seats here
https://www.youtube.com/watch?v=AOhUYIG6Qv4
The easier is to replace the whole foam/cover part, the bottom one should be doable but the hard part is attaching the cushion spring back on the frame
This is my swap (I recommend it on 2001-2003)
https://www.youtube.com/watch?v=oGtD-skiJcc and they way I attached the foam spring back to the frame
https://www.youtube.com/watch?v=fLRsCe-HeXQ