Thanks OP for this post!
I have been able to do this twice now, each time with in-store pick up. I'll try to explain the steps below, hopefully it helps and clarifies things a bit for those who are trying store pickup.
1. Added an additional individual line with an iPhone 6 (line #2) to my individual line with iPhone 5s on UDP (line #1)
- went through the web portal, chose the 450 minute plan --> iPhone unlimited data --> 250 text messaging plan (also a grandfathered feature)
- at the store, the rep just scanned the device and activated the new UDP line, without opening up the new device
- after i left the store, line #2 is shown activated on my Verizon account, with the unlimited data and 250 text features on the line, but without a device attached to it
- went home and used line #1's SIM to turn on and activate the new iPhone 6
- used line #2's SIM in the 5s, waited a bit for it to activate and receive signal
- swapped the SIM cards back, and both lines still showing unlimited data
before going through the in-store pick up process again, I converted the 2 individual lines on the account into a 700-minute family plan.
2. Added an additional line to the family plan with a 6+ (line #3)
- web portal didn't ask me to choose minutes, only data + text features. chose iPhone unlimited & 250 text again
- went through the same steps as #1: picking up the phone in store with a line activated without a device attached to it, then activating the device at home
Curious, would anyone here be interested in taking over one of the unlimited lines (for free, contract end date 9/23/2016)?