Originally Posted by JessicaB

Just to add (as I had exactly this situation earlier in the year), if you do want to do the same route as originally booked, you DON'T need to have Avios Reward availability in J, there just needs to be a seat in J available on the date that you want to fly. BA are incredibly generous with this, you had to have Avios availability for the original flight to be able to upgrade, but when it's medical they let you pick any flight which has a seat in the J cabin. When it happened to me the agent had to check with a manager on the rules but confirmed that this was the case.