Really positive news in general. However!
1. It does concern me regarding catering - because BA can barely cover that for existing loads.
2. The cost not being in-line with other Avios upgrades just seems an oddity. Why not just leave an Avios seat open until the day or departure, then permit in-person upgrades last second if it's still there?
Hopefully they'll see the necessary feedback in the four-week trial to get it right. Because in the future this could be very useful, just as it is with Virgin Atlantic.