Here's what Apple says, which I know does not completely answer your question:
Downgrade or cancel your iCloud storage plan - Apple Support
"What happens when you decrease your storage plan
When you downgrade your iCloud storage plan, the storage decrease takes effect after your current monthly or annual subscription ends.
If you downgrade or cancel your plan and your stored iCloud content exceeds your available storage, new photos and videos don’t upload to iCloud Photos, files don't upload in iCloud Drive, Messages in iCloud don't update, and your iOS devices don't back up. Learn how to manage your storage by deleting the information that you don't need anymore.
If you downgrade to the free iCloud Storage plan, you'll have 5GB of storage."