I've always preferred the bmw alloy luggage on the gs to any other,so when I swapped my TC ,on which I had fitted full bmw alloy luggage, for the new wc I was told that none of the luggage would fit.
I could not get the panniers to fit,however it was a very simple task using nothing more than some stainless flat bar,a hacksaw, various drills,a vice,and a dremmel to fabricate some brackets to make the old rear rack fit the new bike.
I'm no fabricator,but it took me about 4hrs and cost less than £10,plus imho it looks better than any other top box I've seen.
So,in answer to your question if you use the old style adventure rear rack then yes fitting the old top box to the new bike is easy.
I'm assuming that the rear end of the gsa wc will be the same as the gs wc which I'm sure it will be.
Steve