We know from 1.06: Skin that shapeshifters can absorb the memories of the person they turn into - the joys, the sorrows, the little resentments, everything. So - what would happen if a shapeshifter attempted shifting into Resouled!Sam? I would imagine the 'shifter wouldn't have the luxury of Sam's Wall, so there's nothing to hold back a century or more of Hell. Would it break this 'shifter? Would it be reduced to a gibbering mess? Or worse?
And to up the