See the following Apple Support article for information on sharing iCloud storage: Share iCloud+ with your family - Apple Support
There is no data to migrate. You choose to share, she accepts. Her storage stays where it is and continues to be accessed in the same way via her Apple ID; only the billing total and the amount of available storage change.
Also note the following from that support article:
If a family member is already paying for their own iCloud+Â 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.