Sharing my iCloud plan with daughter who also has a plan?

She won't lose her data. She will continue to access her data using her own Apple ID and she can join your plan as described in the following from Share iCloud+ with your family - Apple Support

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.

See the full instructions in the "Share iCloud+ with everyone in your family" and "Switch from your personal iCloud+ plan to your family plan" sections of Share iCloud+ with your family - Apple Support

chaznsc Said:

"Sharing my iCloud plan with daughter who also has a plan?: My daughter has an iPhone and an iPad. I have an iPad. We both have separate 200gb paid accounts but need more storage. I’m considering getting the two terabyte plan and sharing with her. If I do that will she lose her data in her current plan? Not sure I’m asking this correctly but it’s a bit confusing."

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Sharing iCloud Plans:

iCloud Accounts are not merged. They are shared. So, if you stare your subscription, she can use your plan, and keep her's at the same time.

1. Go Here: Share iCloud+ with your Family - Apple Support
2. Scroll to: Switch from your personal iCloud+ plan to your family plan
3. As it Reads: "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."

chaznsc Said:

"I JUST NEED TO CONFIRM IF SHE LEAVES HER PLAN WHAT HAPPENS TO HER OLD DATA?"

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She'll keep the Data.

Download All of iCloud:

If you are oh so concerned, then have her download all of iCloud to an external drive, prior to performing all of this/ Use my user tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip

chaznsc Said:

"Thank you all for the asssitance. We made the jump and everyone made it!"

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You are welcome! Excellent job troubleshooting!

Backup Every-So-Often:

For the record, keep a backup of your iCloud Drive, every-so-often, as a way to recover data: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip