Try using a bit of heat from the back side, to loosen the glue. Not too much.
A hair dryer on low for a bit. Thanks. If you think it's going to get destroyed, leave it.
You may want that on there, for warranty work.
What's the serial # on that 12H? There where 2 different seat pans used. 37/38 or 52.
I know I have some 52s. They'll need new cushions of course. Might have a 37/38. I'll look