I’m pretty sure you have to cancel the order and pre-order again.
I spoke with a Verizon rep about changing the email address for my ordered and worried it'd change my ship date and he said it wouldn't because I already placed the order. However, he said if I wanted to change the billing or shipping address that they'd have to cancel the order and replace it therefore giving you a later delivery date.
Ugh that sucks. I don't know why it wouldn't let me change the shipping during checkout. It did during my test runs with an 8.
Oh well.