You only need to meet the requirements when clicking the "Upgrade" button for the first time for the level that you're aiming for. If you get downgraded, you're free to upgrade back without chasing the same ol' requirements.

However, you'll need the requirements for the levels not yet reached, so it's usually best to plan upgrades from one milestone level to another. That way, you don't have to chase bigger requirements from scratch. It's also wise to consider boosting these upgrades. It can be easier and cheaper than defending.

To make it clear, let's come up with an example. Say you have reached level 8. For level 9, you need to meet the requirements. You do that, and now you're level 9. You drop Presidios. If you get downgraded to 8, you can upgrade again. No problem.

But you'll still be open to attacks and downgrades, and you'll need to keep defending. So, you'll want to get to level 12 (a milestone level). But, that will have steeper requirements than 9, meaning that - to reach level 12 - you need to go above the requirements for 9, after you dropped them below that level.

So, best to not go below requirements for 9 anyway. Instead, get the ball rolling and keep upgrading from 8 to 12 (not just to 9) and then either continue the upgrades or disband Bastion def and Presidios that you don't want to keep.

I hope this is clear enough. If not, ask.