Originally Posted by East_and_West

Depends on your ticket type. If it's a Flex one, and you have unused upgrade vouchers, you can call A3 CS and request an upgrade. If there is I class availability, you (usually) get upgraded immediately; otherwise you are waitlisted. If your ticket is a GoLight one on U, P or T fare, then it is not voucher-upgradeable. What you could then is purchase an upgrade using M&B miles, but I think that this is subject to the same availability as the voucher upgrades (I think both come out of the I bucket, but I am not sure since I have never purchased an upgrade this way... In most cases I have found it to be a rather low value-for-miles). Alternatively, if you want to fly Business really badly, you can also try offering a bid online through the A3 app or their website, but this is of course not complimentary.

Note that in either case accrued mileage will be according to the purchased Econ fare bucket (P/U) and not the upgraded/Business one.

There is of course always a chance that you may receive an op-up at the gate, but this is a really slim one and definitely not something you should bet on.