I had initially thought I'd just disconnect the boat from the scaffold before putting on the bottom ply skin, and the weight would keep it in place, then I'd lift it off the scaffold and turn it.
While looking for all the things I'd want to do before putting the bottom on, it dawned on me that the frame that goes through the scaffold, second from the front in the picture opposite, is going to be a problem.
My scaffold is spiked into the ground. So I think that I will have to reach under the boat and cut the scaffold legs, then turn the whole boat scaffold and all and take apart the scaffold after the turn.
I'm glad I am trying to figure this out now, not later.