Just did a little more checking...
AmazonWireless
I need a new phone, but my line isn't eligible for an upgrade. Another line in my family account is eligible. Can I get a new phone for my line by using the upgrade of the other line in my family account?
While carriers may offer the option to swap upgrades between different lines on the same family account, this option isn't available through AmazonWireless. Phones purchased from AmazonWireless must remain on the line upgraded for 181 days. If you move the device to an alternate line within the first 181 days, you'd be subject to additional charges by AmazonWireless of up to $250 (per phone), plus applicable taxes.