I mean the house of wisdom. Just like the Chinese can build both landmarks, the Ayyubids should be able to reinforce their already build wings to the next age if they pay the cost again.
I’m not coming from a balance point of view, but from a “reduce excessive complexity” one. Whenever I choose a Wing I not only have to weigh it against both the remaining options and the 2nd choice, but I also have to consider missing out on the later buffs. If I could upgrade an already build wing, then at least I wouldn’t have to stress about missing out on those.
And given that they aren’t even visible as a tooltip, that seems to demand a rather high amount of knowledge. Things like villager production and always on healing come out of nowhere imho.