what an interesting question. In my own case - well actuall I only had one partner so its not been tested
Both happen of course - but then we only really hear about the successful switches. Interesting how the top dancers, at least in Standard, seem to have made all the switches and exchanges possible. But there are innumerable cases where changes either led to improvements only for one partner or both failed. In the canadian dance scene I know of one couple that split, tried out with everyone they could and then went back together (and it still didn't work).