What are the benefits of the shared account? I don't want to jeopardize my unlimited account.
Dude get her off of mommy's account and go with a shared account. You get the new phone every year and she gets the hand me down. And yes, ATT has a link to transfer upgrade eligibility between the phones. But if you go with ATT Next, it's no longer an issue as both can get upgrades every twelve months (not that she'll want it)...