If you are 'repairing' your fort after an attack you should not need to downgrade an enemy fortress

If you have downgraded an enemy fort, but then your own fort is downgraded, then you will lose the credited downgrade, but can still 'repair' your own fort

Repairing and Upgrading are treated differently

https://plariumsupport.zendesk.com/hc/en-us/articles/115002249025-Though-we-downgraded-the-hostile-League-Fortress-we-cannot-upgrade-our-Forstress-to-the-next-level-

Though we downgraded the hostile League Fortress, we cannot upgrade our Forstress to the next level.

First of all, please check if the downgraded enemy Fortress matched the level, specified in the upgrade requirements. These requirements can be found in the League Fortress window.

Also, please, make sure that when the battle at a rival Fortress took place, your own Fortress 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 Fortress level changes, the old requirements disappear, since they are substituted with the new ones. If the downgrade requirement was not fulfilled, then it is most likely that you did not have it in your Fortress, when the battle took place.

In other words, when you downgrade an enemy Fortress to fulfill the level 9 upgrade requirements, your own Fortress should necessarily be on level 8 at the time of the time of the downgrade.

Nevertheless, you only need to complete Fortress downgrade requirement once on each level. Therefore, if your Fortress was downgraded, you will not have to downgrade a rival League Fortress again to bring your own Fortress back to it's previous level.