Lets say downgrading is possible. (I cant think of a reason why it could not be, but it is not recommended)
How are you trying to downgrade the board? With the arduino IDE or via UEFI shell ? Galileo Gen 1 might be theoretically be able to be downgraded.
I would place the best bet on downgrading via UEFI shell.
Make sure you have the correct files and try upgrading the firmware via Serial in the BSP build guide.