I'm not sure of the exact details, but there is a grace period if you have a delay in getting service at your new place.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Hello CBou, I can address you concerns for you regarding your ETF. We will honor waiving any ETF if you remain a customer with Comcast. There are cases like yours where a customer can't restart services right away and we later on waive the early termination fee for them. If you move to an area that is not serviced by Comcast and will not be in the future then the ETF would be valid and we would not be able to credit the fee. If you'd like I can track this for you and request for the fee to be credited once you restart services. Please send me a private message with your full name, street address, and account number by clicking on my name (ComcastAmir) and then "private message me" if you would like a follow up.