First, please check if the downgraded enemy Coalition Capital matched the level specified in the upgrade requirements, which can be found in the Coalition Capital window. Also, please make sure that when the battle at a rival Fortress took place, your own Capital was on the appropriate level.
Like in every other quest or event, it is necessary to have the upgrade requirements present on the list, to be able to fulfill them.
Whenever your Coalition Capital level changes, the old requirements disappear since they are replaced with the new ones. If the downgrade requirement was not fulfilled, then it is most likely that you did not have it in your Capital when the battle took place.
In other words, when you downgrade an enemy Coalition Capital to fulfill the level 9 upgrade requirements, your own Capital should necessarily be on level 8 at the time of the moment of the downgrade.
Nevertheless, you only need to complete the Capital downgrade requirement once on each level. Therefore, if your Capital was downgraded, you will not have to downgrade a rival Coalition Capital again to bring your own Capital back to its previous level.