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The only way that would work is if you sell your phone, your active sim card, and don't really mind the phone line still being finaincially responsible in your name.
I don't think anybody should put that much trust in a complete stranger because there's no telling if they'll go over their minutes or rack up any other types of overages/usage charges that you'd still be responsible for in case the person you sold the phone/active sim card to decides to not pay the bill.
If you decide to do a transfer of billing responsibility by transferring the line into a new person's name then you forfeit your grandfathered status and lose the unlimited plan... Making the entire transaction pointless...
Sounds like a lose-lose to me.