Can a family member who already has an individual account transfer their play lists, library etc.?
Can a family member transfer their playlists, library, etc to a family membership?
Question marked as
Top-ranking reply
There is no need to transfer. Once the family member joins the family plan, the individual subscription will be automatically cancelled and his music library will remain intact. See the footnote in the 'First, set up Family Sharing' section of:
Get an Apple Music family subscription - Apple Support
If he cancels his subscription before joining, he may lose his music library.