So long as you have another line to upgrade. Cancelling now will result in a 48 hold before it is effectively canceled. Trust me I just called and asked my local store earlier, though luckily for me I have another line I can upgrade, then will cancel my order online and move the phone to the right number.
Not sure why your local store told you that but it's not true. I know from personal experience with iPhone pre-order being canceled over the phone while in the store and having them verify the cancelation so they can complete the in store sale. They may be trying to discourage people from doing it but the wait is NOT 48 hours. We have done it this way on previous pre-orders and walked out of the store with a device. As long as the order hasn't been completed (labeled and waiting to ship) then there should be no issue and no hold. Just don't cancel before going in the store.
Maybe it changed? Maybe that's still true. Guess I need to wait till tomorrow to know for sure, but when I called he plugged my number into the system and he said he couldn't do anything because there was a hold and a 48 hour wait time. Does seem like he could have been trying to discourage me from cancelling my online order so I simply replied with the answer of upgrading a different line and waiting the 48 hours.