talk to the plan admin in the company
Each line of service has a contract of it's own. The contract is for 24 months, after which the line reverts to month-to-month. You are never DUE for an upgrade. You are eligible for a subsidized upgrade at the end of 20 or 24 months, depending on when your last contract was signed. Yes, it is possible to use someone else's upgrade eligibility to purchase a new phone and then put the phone on your line of service. However, you have now obligated that other person to make payments for another 24 months. I don't know why anyone would allow you to use their upgrade eligibility and obligate them for 24 months of payments. If you were to do this, your line upgrade eligibility would not be affected.