My wife and I had iPhone 4Ss on Verizon and we wanted to keep our service. Here is our process/timeline:

12/27/2015 – Applied
12/29/2015 – Approved (I called Ms. Ike did not)
1/4/2016 – Requested Nano SIM from Verizon via chat.
1/8/2016 – Cards arrived in the mail, ordered iPhones
1/8/2016 - Signed up for Plenti (Ike)
1/9/2016 – Nano SIMS arrived in the mail
1/9/2016 – Faxed receipts to Citi (1-866-799-5591)
1/11/2016 – Email from AT&T that Web Order “needs further action.” Called, approved on phone.
1/12/2016 – New iPhones arrived
1/12/2016 – Sent unlock requests via web
1/12/2016 – Received email to acknowledge unlock request followed by confirmation that phone was unlocked. Check status of unlock request at: https://www.att.com/deviceunlock/status.html#/
1/12/2016 – Backed up old phone in iTunes, installed Verizon Nano SIM, turned on phone, and loaded from iTunes backup (if during load you are asked about changing carriers and downloading software, answer YES!)
1/12/2016 – Chatted with Verizon online chat and had them register IMEI. Phones worked (had to have both old and new phone powered off).
1/14/2016 - First billing cycle closed for me, I was short due to a charge taking 3+ days to clear.
1/20/2016 – Signed up for AT&T online accounts.
1/20/2016 – Signed up for Plenti (Ms. Ike)
1/20/2016 – Linked Plenti accounts
1/25/2016 – First billing cycle closed for Ms. Ike, met spend, no credit.
1/27/2016 – Called Sprint and cancelled service, requested pro-rated bill was told not to pay existing bill and prorated bill would arrive by 2/19/2016.
2/1/2016? – Plenti points showed up in both accounts
2/12/2016 - Second billing cycle closed for me.
2/13/2016 - $649.99 credit appeared (Ike).
2/21/2016 - Got bills with same charges ($70.63) plus $5.01 late fee. Called AT&T, after three attempts got CSR to remove $5 fee but said other charges were legitimate. I told him that I only had the service for 15 days so the bill should be prorated, he then removed ALL charges on both accounts.

Phone cost: $649.99 iPhone 6+ (-$649.99), $39.00 tax = $39.00
Credit card cost: $95.00 annual fee
AT&T Service: $70.63 (-$70.63) = $0
Sold iPhone 4S on Craigslist: -$60.

So net $74 for new iPhone 6+ for me and if Ms. Ike's credit appears in a few days I can cancel her card and get back the $95 annual fee so we would make $21 on her phone.

A couple of notes in case anyone missed them in earlier posts.
1. Phones are activated and billing begins the day you purchase the phone. They pro-rate 2 days prior to service starting. Not when you receive phone. Not when you activate phone. This is shady.
2. You do not need to ever have the phone on AT&T service in order to get the credit.
3. I decided not to test cancelling service prior to 15 days as the risk was too high and it ended up working out for me anyway.

Thanks to everyone who contributed to this thread. Ms. Ike is very happy with her new phone and I am very happy not to have paid hundreds of dollars for it!