Can someone explain, how the new "miners can volunteer to raise their share difficulty"-protocol change works? I don't understand how cheating is prevented.

Can't you just mine in secret and for example find a difficulty 1000 share, and then connect to the p2pool network and send: "hey, my address is 1abcsomething and I'm mining difficulty 1000 and here is a share"? And then afterwards mine in secret a difficulty 650 share and connect to the p2pool network and send: "My address is 1xyzsomethingdifferent and I'm mining difficulty 650 and here is a share"?

How can you define a difficulty in advance, if you can simply reconnect with a new identity?

I'm sure I'm missing something here. Please enlighten me