From what I've gathered (and I switched my upgrade to another line) it's much easier than some VZW CSR's make it. When I bought my Maxx on launch day, they told me I needed to get a nano sim with NFC. Unfortunately they didn't have any in store because this is the only phone that has a nano NFC card (iphone has nano but not with nfc). I was told wait 7-10 days and they MIGHT get some in or I could call Verizon myself and order one. After calling EVERY Verizon store in Cincinnati I found out that none of them had the new nano nfc cards because the Maxx came with one in it and they didn't anticipate needing one on launch day. So I called and explained my situation to Verizon, the guy said no problem he'd send me one. Two days later, when FedEx arrived, they delivered two standard sim cards.
A user at androidforums.com in the Droid Maxx Forum explained that he is a Verizon Wireless Customer Service Representative and that any Verizon store that still have computers (rather than iPads) could easily reassign my original nano to my phone number from my son's phone number. I called Verizon Wireless, explained my frustration and the CSR that I spoke to made the change by phone in 15 minutes.
Long story, short point . . . transferring your upgrade is much easier when you know what to expect and what to ask for.