Apple ID

One of you'll keep the current one as your own. Other person signs out of the Apple ID on all their devices and chooses the option to keep content on the device, if asked, then they will create their own Apple ID and sign into it and turn on their preferred iCloud options again.

Lastly, the original Apple ID will send a Family Share invite to the new ID. https://support.apple.com/en-us/HT201088

See the Apple Support article(s) below.

Apple ID - create a new.      

Apple ID – Create        

Apple ID - Could Not Create or Could Not Set Up iCloud message.