But a word of warning: don't do this if you aren't sure, you will be making irreversible modifications to the MY seat frames and if you back out half way through, you'll be stuck with no seats. Also, once the L seats are in they are a lot lower than the stock seats, this alters driving position and view out the front of the car.
So, to start, take the MY seats out of the car, we will be prepping them first.

Take the back rests off of the MY seats so you are left with the base and runners. You can discard the back if you want or keep it. Flip over the base and cut all of the metal rings holding the vinyl seat covering onto the frame, and peel back the covering on all 4 sides so you are left with:

Still on the underside, you will see all of the seat springs embedded in the foam and attached to the frame. We need to remove the old cushion and I found the easiest way was to Cut all the springs I could see until it all came loose. Be careful not to set the foam on fire if you use an angle grinder.

After you've cut the springs, grab the front of the cushion with one hand and stand on the frame, pull back the cushion tearing it from the frame until you get to the back, where the springs again attach to the frame. Cut the springs again and you should be able to cut or tear the whole cushion free. Use an old bread knife to clean up as much foam as you can. You should be left with a clean frame.

