you saw correct, 2.8 can not be downgraded... so once you go to the light side, you can't come back to the dark.
The closest you can get to running homebrew is 2.71 which can emulate 1.5 firmware (should you want to). I wasn't aware that the demo requires 2.8. Thats silly since the game requires 2.71 (if you were to buy it).
I guess it's sonys way of forcing everyone to stay upgraded.