I'm moving from Sprint to AT&T primarily because I realized that the difference in price between subsidized phone (on AT&T) and unsubsidized (on Sprint, because I'm mid-contract), even considering my ETF, is actually larger than the difference in AT&T and Sprint monthly pricing.
And I'd like to do what the OP wants to do. And ideally still be able to preorder with Apple at 12:01!
My other option would be canceling Sprint and pre-ordering for a new phone number on friday.