You might think that thru. You won't be able to cancel until you receive the phone. If you can't cancel, you can't get a phone at the store - your number is already marked for the upgrade. Typically the in stock at stores dry up for a bit after opening day (although the Xs, Xr thing might make things different this year). And if you have IUP, you already had your credit check.
You are trying to speak rationally to irrational thoughts...the world hopes someone listens.