I tried this and it failed. They would not let me do this. It was a BS reason. But it's a real hassle to return. They had to call in manually, wait on hold for awhile, and then they did not refund all of the money. They seemed not to refund a $36 upgrade fee. Just so you are warned.
That said, there are no rules. I'm sure you can get things done if you are persistent enough. No one inside Apple has any definitive word and stores do whatever they want to do.
fyi On a new line reservation, I was not charged with any activation fee whatever maybe that's the difference, you reserved as an upgrade.