Talk:Alliance/@comment-86.56.52.60-20130527234907/@comment-7513606-20130528024349

At the very bottom of the in-game Defenders page states ths:

"If you change your alliance before Jun. 3rd 01:00 am (ET), you will become unable to pick up your alliance ranking rewards. Please be careful."

So, it seems that at least during this event, if you leave an alliance, you will not be eligible for any alliance rewards.