I was talking with A3 yesterday to upgrade an upcoming flight but it had to be put on the waiting list and an interesting thing came up.
The agent informed me that as of recently you can now use your upgrade at the check-in desks regardless of I class availability as long as there are any business class seats available on the flight. GoLight U,T,P fares are still excluded though.
Quite useful when I think of some times that my upgrades haven't cleared but business class was not full ^