Good evening JH133,
Welcome to the Apple Support Communities!
I understand you have an individual Apple Music subscription and want to know what happens if you change to a Family plan. The content of the Apple Music library for your Apple ID will not change. Other family members will sign up with their own Apple ID so their personal music library will remain the same as well. You can share playlists or music you've purchased from the iTunes Store with other family members if you wish. I'll include some information about this below:
Get an Apple Music Family subscription
Share App Store, iTunes Store, and Apple Books purchases with Family Sharing
Create and share playlists
Take care.