Transferring Upgrade Eligibility with Phone Bought from External Retailer

I have two phones on the same Mobile Share plan:

Phone A: Basic Phone, non-full upgrade eligible

Phone B: Basic Phone, full upgrade eligible

I would like to preorder a Galaxy S6 for Phone A and put it on AT&T Next, using Phone B's upgrade eligibility.  There would be no issue doing this via AT&T's store, either online or real.

The twist is that I would like to take advantage of Best Buy's offer of a free charger with preorder.  Best Buy's website does not support eligibility transfer.  It looks like I can order for Phone B using Phone B's upgrade eligibility only.

I have a hunch, which I'm looking to confirm here, that the general mechanics of the purchase and eligibility transfer are the same regardless of whether the phone is bought from Best Buy or directly from AT&T, i.e.:

1. Buy new phone for Phone B
2. Receive new Phone
3. Contact AT&T, either by call or in store, to have the SIMs/phone numbers/plan type assigned as I want them (new Galaxy S6 assigned to Phone A and Phone B remains a basic phone)

Do you see any issue with purchasing the new phone for Phone B from Best Buy and then having the two Phones' eligibilities swapped when I receive the new phone?

Thanks in advance for insight or advice anyone can offer.