If you are mining I recommend you wait until your next P2Pool payout before switch to new server. Otherwise you lose any work credit since your last payout. This is special true for small miners with long payout interval.
Is this necessarily true? I thought p2pool was smart enough to payout no matter where you are hashing, it's just that the graphs would look inaccurate. Graphs only show the payout for the address that was mined on that specific server, but the payout you receive is for what you earned anywhere in the world on p2pool.
After more code studying I see you are correct. The accepted work of miners is only used for graphs. This means there is no reason for miners to continue mining on the old server. No work is lost by switching now.
Sorry for posting misinformation.