Upgrade Swap Question

I have a question about contract lengths in regards to cross-grading/upgrade swap.

Let's say that line A and line B are in a family plan. Line A's contract ends in 180 days, while B is already eligible to upgrade:

A used B's upgrade and bought a new phone. What if B decided they wanted to finally upgrade 180 days later, would B be able to use A's original upgrade, or would this be lost and both A and B would have 2 year contracts renewed?

I was thinking that B would have an upgrade available whether the contracts were actually switched or B would need to do what A did, but vice-versa to get a new phone, but I just wanted to check before doing the upgrade-swap...

Basically, A has a phone in mind, and B wants to wait for the newer version to be released of a different phone.

Thanks!