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« on: May 27, 2010, 10:32 PM »
Your number CANNOT exist with two carriers at the same time.
Exactly how the transition takes place is really up to the new carrier (what their procedure is).
I would suspect (I would like to hope) they provide you the new phone and then give you an option to determine/control when you have the number ported. The actual process of porting the number takes only a few minutes.
HTH