Hi,
My inclination would be to try it in your order and see if it works. If it does then great. If it doesn't then maybe give Apple a call and ask them if there's a way to make it work.
This is what Apple says about downgrading:
Downgrade or cancel your iCloud+ plan - Apple Support
What happens when you downgrade or cancel iCloud+
- If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information. iCloud updates again when you increase your storage capacity or delete information to make more space.
- If you downgrade to the free iCloud plan, you will no longer have access to iCloud+ features like Hide My Email, Private Relay, and HomeKit Secure Video support.2
- If you pay for both Apple One and an iCloud+ plan, you can downgrade or cancel the iCloud+ plan and just use the iCloud+ plan included in Apple One.
I note that it doesn't say it will actually delete items, it will just stop you from adding anything more. In that case if you downgrade and then upgrade all should be as it was. I know that observing the phrasing of Apple documents does not make for a comfortable feeling so again, maybe ask Apple since their documentation does not make this clear.
Oh, and when you find out, please post here so I know the answer. You're not the first to have asked. ;-)
Contact Apple. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support
