Originally Posted by jerry a. laska

If it is an AA issued ticket then you cannot use avios to upgrade (you could use aadvantage miles to upgrade if booked in these classes: C, J, D or R on British Airways).

If it is a BA issued ticket with an AA flight then you can use avios to upgrade these fare classes: J, C, D, W, Y, B. Since your ticket is I class you could not use avios to upgrade the AA flight.

If it is a BA issued ticket with BA flights then you could use avios to upgrade.