In Downgrade or cancel your iCloud+ plan - Apple Support Apple says: "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."
So my understanding from that is if iClould+ is temporarily turned off for everybody, no data are deleted, everything is in stasis and once a member in the new group has a subscription at the family level then other family members are covered again.
However, I don't know about the other features. "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." I know that if this was Apple Music then even if your subscription coverage lapses for a few minutes, playlists are lost.
So maybe nothing lost if it is just 5 minutes while changing family groups?
The other way is to play it safe and for everybody to take out one month personal iCloud+ plans to bridge them over the switch.
Or ask Apple and see what they think (and let us know). Refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
This page has telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support