What info do I need from the primary account holder to upgrade my phone?

If you're buying a phone using eip then you're basically taking out a loan for the phone and the account holder is obligated to make sure the phone is paid off.

No loan. I’ll be paying in full, with no trade-in.

You need to have the PAH change you're line settings then.

The PAH can do that in their Mytmobile account on line permissions.