1. We all know Logic Board KEY Marrying with the Nand Memory.
2. Downgrading will disable BD Playback, why? Maybe while downgrading, the previous KEY in Nand Memory is erased. So, after downgrading, the Nand do not have the previous KEY. So now the BD ROM is DEAD.
So, maybe the solution is :-
1. Swap the Logic Board Nand Memory (KEY) from dead BD ROM to New BD ROM.
2. Some one can extract the KEY from Previous Logic Board than flash it to New Logic Board.
3. Or extract the KEY from Nand Memory, like homebrew software that can access the Nand, like Comginies than flash to the new Logic Board.