This is completely backwards. Straight from Verizon (using your example)...
Your first sentence is accurate. Where the information becomes inaccurate is in the second sentence.
If you use line A which has an upgrade available to purchase a phone at the upgrade price, line A will now have the newer contract and will have to wait 2 years to terminate without an early termination fee. It doesn't matter what phone line or number that new phone is ported to, all that matters is that line A was the line used to effect that upgrade and so line A will now be on the new two year contact.
Now, you can move phones from one line to another by swapping ESNs (MEID). I did this with my wife's phone. She had the same number for over 15 years (sane as me), but the line she was on wasn't eligible for an upgrade. At that time, Amazon was running the .01 cent sale on RAZRs, so I opened a new line on my family plan for her new phone, adding a new two year contract with the double data plan (4GB for the 2GB price . Then I swapped that phone over to her grandfathered unlimited data plan, stuck my OG Droid on that line to continue the obligation and now my wife has a RAZR, same old phone number and her number is still grandfathered for unlimited data, and is not under the new 2-year agreement, the new phone number with the OGD is. Also, I used the available upgrade on one other line I had that was out of contract to subsidize my RAZR purchase and again, I am still grandfathered and beyond my contract cutoff date Meanwhile the line I used for that upgrade is now under a new two-year agreement.
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