I caught grief when I tried to downgrade from the $2.99 to 99¢ iCloud plan, from my iPad. The “Done” button remained stubbornly grayed out, leaving the live “Cancel” button as my only option. The fix follows.
Fix: choose “Free.” Click Done. Back out of Settings completely. Wait a few minutes. Go back in, and change from free to the 99¢ plan. Finally, the Done button goes live, allowing the change to stick.
This will shock ya– no such hassles encountered when upgrading. 😏
Learn about the iCloud storage plans that you can choose from, how to buy more storage or downgrade your plan, and where you can change your payment information.
This fix appears nowhere on the official page. But it’s available for those who care.
Source: iCloud storage upgrades and downgrades – Apple Support