Originally Posted by East_and_West

No, generic C-availability is not sufficient. There must be availability on booking class I to get an instantaneous upgrade. Otherwise, and provided that there is still a sufficient number of business class seats unsold, additional I-class fares are released about 48 hours prior to departure. But this is not an option for you given that your vouchers expire before then.

Notice also that if you upgrade a flight, then you can no longer make any rebookings online, even if it's a flex ticket. Any changes should go through their call center which entails a €20 fee (there is currently a corona waiver, but I don't know if this will still be the case in summer or fall). Even if it is, at the time when you rebook there will need to be I bucket availability in the flight which you would like to rebook into in order for the upgrade to carry over; otherwise your voucher will be simply lost.