Carpetman, update for you. You can change the reservoir without draining the tank but DON'T!
It would take two people and still the amount spilled and the resulting mess you have to clean up is not worth it. Some quick steps for you. Sorry no photos, I was too busy with the insane amount of mess to deal with.
1) Raise car and remove pan-8mm
2) Find red drain cock on the left side bottom of radiator
3) On the side closest to engine, there is a spout to connect a tube to. Connect tube on the port and other side in container large enough to hold the engine coolant. Use flat head to open drain cock. Then open the pressure top on the coolant expansion tank. Close drain cock when finished. Tighten pressure cap on expansion tank. This will help cut down on spillage later
4) Once drained, remove two nuts holing tank in place. 10mm
5) Remove overflow hose from port on top of expansion tank. Screwdriver
6) Remove electrical connector from sensor at the bottom of the tank.
7) Remove hose from bottom of the expansion tank. Screwdriver. Be prepared to catch the remaining coolant that will spill out. Large shop rag will do
8) Pull tank straight up. It will require a good hard pull to get the retainer to pull free from the grommet. Watch where your hands are located because when it comes free you will bang your hands on anything in the way. Also do not bend over the tank while pulling up, you may hit yourself in the face with the tank
9) Turn tank over and remove sensor. Place sensor in new tank.
10) Set tank down where it will reside but do not push it in place. Re-attach the bottom tube. The reason you want to sit the tank in place before attaching the tube is to make sure the tube does not twist and kink.
11) Once tube is attached, push tank into place
12) Replace nuts to hold tank in place
13) Replace electrical connection to sensor
14) Attach overflow tube to port.
15) Remove the tube that hangs down next to the pressure top from the old tank and place it in the proper spot on new tank.
16) Refill tank and check for leaks.
It has been reported that you massage the bottom tube to work out the air bubbles when refilling. I have not gotten to this step yet because I had planned on reusing the old/new coolant that was just placed in there. However there was so much debris in the bottom of the expansion tank, now I am thinking I need to do a flush and start from scratch.