I probably will just put the money on my account the day before, I have the money to pay for it anyway so it's no big deal.
It may not be that easy due to the new rules but i would call the store you reserved it to go to and see what the store manager says. That way u know for sure. I don't think you should have a problem doing this tho. If it's worked in the past then it should work again.
I'm exactly on the same situation as you, checked at the local store, guy there showed me what was currently available for me if I changed phone now (no iphone 6 yet in computer) and I had choice for an iPhone 5S for 3 years (79.00$, corp plan), 2 years (99.99$) of full phone price. I'll also be able keep my current plan at 48$ + taxes.