Buzz’ AOC was valid for the 733 and the 146. This license is only valid while Ryanair operates those planes. When Ryanair retires them the license will loose its validity after I believe 1 year or so.

I think the reason Ryanair holds on to its Buzz license (at least for now) is so it can operate the 733. After all, Ryanair still uses at least some of the Buzz 73s.

If Ryanair had cancelled the Buzz AOC it would have to extend its own AOC to include the 733. This likely won’t be a problem but does cost some money. Since Ryanair already has a valid license that costs nothing to keep Ryanair would be silly not to use it.