Also I wonder if legacy positions are being wound down to give Greg more degrees of freedom

I think that extremely unlikely.
I'm sure current management won't mind if future management restructures things (just as Mr Buffett promptly and unemotionally ditched the entire portfolio of Lou Simpson on his departure), but the decision will be up to new management.

No...if Berkshire is selling something now, it's because Mr B thinks it's a good idea to sell it, for whatever reason.

On that, we can only speculate. I suspect it isn't simply the current high-ish price level, as the price was higher two years ago and no move was made, and in any case a temporarily high price normally isn't nearly enough to get Mr Buffett to change a position that he still likes for the long haul. I would therefore speculate further than think he smells a risk in the air, but of what type I can't imagine.

Jim