Currently we can only discuss what happened with previous iPhone pre-orders.
For Apple pre-orders the IMEI an the ICCID of the SIM are parked on the line that ordered an upgrade. When you attempt to activate, you will not have any option to choose a different phone number by default.
For AT&T orders the IMEI and the ICCID of the SIM are also parked on the line but AT&T has a different activation process. You call a special activation number and answer VRU responses to complete the activation steps.
With help from AT&T, you can get the ICCID of the SIM and the IMEI moved to another line of service. This can be done by calling Customer Care or by going to an AT&T Corp retail location.
In some cases AT&T reps are available in Apple retail stores during launch and they can also help fix this issues with an upgrade swap.
Note with the new Nano-SIM, I expect there to be issues like we saw with the iPhone 4 when the Micro-SIM rolled out.
Dave