Traffic goes to the URL on the order. If that URL is wrong, visits sent to it are still visits we sent, and they are neither credited back nor refunded — so read it twice before you send the tape. The same applies while your own site is down, timing out or answering with errors: we cannot see your server from here, and traffic delivered into a broken page is delivered. What you can do, at any point and in writing, is ask for the campaign to be held; it stops, and it resumes when you say the page is back.
Swapping the page a campaign points at, mid-flight, is the one change that needs agreeing first. The order was read against that page, and the refusals below are read against it too.