If I already have a subscription through Apple Music but change to the family plan, will I get charged when I change? Or when I’m originally supposed to be billed?

Hi Pawenski-0,

You started from a great resource. How to switch to a different subscription plan

If you've already set up Family Sharing, once you change your subscription type you'll be billed accordingly as everyone in your family group will automatically have access to Apple Music after. You can also view steps for sharing subscriptions below and in the guide here: Share Apple and App Store subscriptions with family members on iPhone - Apple Support

"Share Apple subscriptions

1. Go to Settings  > [your name] > Family Sharing.
2. Tap Subscriptions, then do one of the following:

• Tap a subscription you want to share, then follow the onscreen instructions.
• Tap Manage Subscriptions, then tap a subscription."

You can also find helpful information here: Get an Apple Music family subscription - Apple Support

Cheers.