Apple One
What do I do if I get a family plan and one of my family members already subscribed to Apple Music, will they need to cancel?
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No. They should not cancel. If they do, they may lose all the music they added to their library from the subscription service.
The Apple One subscription will automatically cancel any individual subscriptions for Family Group members once you get charged for it the first time and will migrate them seamlessly.
click here ➜ Bundle Apple subscriptions with Apple One - Apple Support