Originally Posted by
Xkmo
IAvios upgrades are never available
To upgrade using avios, it's easiest to do at the time of booking.
You need reward availability in the class you want to upgrade to, so booking 6 months ahead at least or ideally 355 days ahead if you want to upgrade to J on a popular route.
Upgrading to proper long haul F is very difficult if you are not BA Gold Guest List (to release an A class F ticket for a reward) because althoigh some J seats are garaunteed to exist for rewards at some point on BA flights, there is no garauntee you will ever see F rewards available.