I have a regular foam mattress with a memory foam topper. It's plush. Memory foam is great in cold weather because it holds body heat. That's not so great in hot weather, like now.
So, after laundering all my bedding today, I decided to flip the mattress and put the topper on the bottom. (It has to go somewhere.) (I know. The topper police will track me down.)
It feels a bit odd not sinking into the mattress, but it still beats a spring mattress. I'll have to see whether the bed is any cooler, and whether the coolness is worth giving up some plushness. It looks like I'm just sleeping. I assure you, though, I'm doing it in the name of science and the advancement of humanity.
UPDATE: It's working out just fine.