I am planning something similar - 2 nights at Poly and 5 nights at BWV. From what I understand on the day you switch over you will have access to both old credits and new credits. From what I also understand, the system will automatically deduct from old credits before deducting from new credits.
But also from what I have read, it is preferable if you can use up your old credits before checking into new resort and can just avoid this. I'm trying to figure it out myself so hoping someone will either confirm or deny my understanding. I'm also unsure how this works when you check in online... do all of your credits just appear at a certain time of day or do you have to physically check in at the resort?