what happens when iCloud downgraded?

When you downgrade any data exceeding 5GB will be deleted. Unfortunately there's no way of determining exactly what data this will be. Other similar services operate an 'oldest out first' policy in this situation, but Apple has never actually stated what they do.

Your only safe option is to go through your data and remove some of it manually until you've got safely below 5GB. As regards email, any messages in the 'iCloud' mailbox are on the server, and there is an 'On My Mac 'section in the mailboxes list (this is on a Mac, I'm not sure how the iPhone app looks).

To download messages, create a new mailbox under 'On My Mac'. Then open the iCloud mailbox, select some messages (don't do a huge number in one go) and drag them to your new mailbox in the list. Wait for them to download - it may take some time.

As per this article https://support.apple.com/en-in/HT207594

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.

You can create a mailbox on the my Mac section https://support.apple.com/en-in/guide/mail/mlhlp1021/mac

And move mails in fractions .

The more refined way for saving mails .

• Take back up of all mails in V6 folder : Method to copy mails in V6 folder .

Click on Go on top menu bar , hold option key click on > library > select mail folder > right click on mail folder there itself > click on copy mail > right click on the desktop > click on paste item .

Entire mails are copied to V6 folder and V6 folder will get saved on the desktop .

Note : For Mac OS High Sierra the folder will be V5 , For Mac OS Sierra the folder will be V4 , For El Capitan - the folder will be V3 , and for OSX Yosemite the folder will be V2 , for latest version Mac OS mojave 10.14.6 the folder will be V6 .

Take a USB stick and copy V6 folder into it , once it is copied right click on the USB stick and eject it .

Also to save the data https://support.apple.com/en-in/HT204055

Any application that is using storage, e.g. Photos is usually the one, then once you downgrade, you will loose data like photos and videos. The same can be said for other applications as well like Mail, Notes, Messages, etc.