I am a diamond league zerg and I decided to try out the "leave league" button just for fun and to see what happens yesterday. If I remember correctly, I did this once in HotS and it just gave me one or two new placement matches and I ended up in the same league as before.
Let me say that my mmr is just diamond, not high diamond, definitely not masters and lol, of course not grandmaster.
That is what happened on my EU LotV account after using the button: I got 5 placement matches, which was a surprise itself. But then something hilarious happened. The first two games were against GM (!) players and the next three against very top masters players. Somehow I managed to beat on master player and got placed in platinum. I thought: "phew, thank god I am back where I belong" but no no! Somehow it screwed my MMR in such a way, that I only play against master players now. I am currently sitting at 1-13.
Can someone explain? Just be cautious guys. I don't know if that was a bug or can be explained in any other way but it definitely screwed me over big time.
Well good luck and have fun laddering.