You may find it takes place automatically. Once you are certain you are being covered by the Apple One subscription you can cancel your personal plan if it is still running.
To find out what happens to your iCloud storage plan when you sign up for Apple One, read this Support article --> What happens to your iCloud storage when you sign up for Apple One - Apple Support
Read this article to learn how to share iCloud+ with your family --> Share iCloud+ with your family - Apple Support
"When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically."
"If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time."