Family plan
right Now 3 family members have their own Apple Music subscription, if we switch to the family plan will we lose the music we already downloaded?
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No. You should not lose anything.
Don't manually cancel each member's personal subscription. Simply invite them to the Family Group, with the Family Subscription, and the transfer should be seamless; automatically cancelling their subscriptions while maintaining their library.
See here: Get an Apple Music Family subscription - Apple Support