Is it possible to use an upgrade from another account?

Yes, you can use her upgrade. The upgrade is tied to her account but you can use the phone that she gets. Depending on how the phone is purchased (usually online or shipped phone), sometimes, it needs to be activated on her account first, as a security, which would make her current SIM deactivated and then she would need a new SIM for her current phone, then you could put your SIM in that phone and it should work.

If she is on the Mobile Share Data Plan, the phone charges would be on her account, if you used NEXT to purchase the phone, or if you got a subsidized phone, she would lose the out of contract discount on her line.

If we're on the same account, what is the difference between this and the Shared Upgrade option? The way you've described is what I've always done, but I was unsure.