Haven't done the upgrade as it wasn't available when I took mine out and refinished.
You have to remove all the water/flushing fittings, pull out the pex just far enough to remove them from their respective lines. Remove all the screws holding the box panels in. Then work a Chinese puzzle to get the panels out of the compartment. Not hard, just be patient in the angling and tilting of the panels. Since the new one is one piece, it could be a more difficult puzzle to get it in place and run the plumbing through the holes.
Edit: I now see the interior panel is screwed in place, so reassembly will be just like removal, in reverse.
"If the women don't find you handsome, they should at least find you handy." - Red Green