Do you lose your music if you switch from Apple Music to Apple One?

No, anything that you have currently that is also included in a plan you select will be a continuous subscription.

Bundle Apple subscriptions with Apple One - https://support.apple.com/HT211659

"If you already subscribe to Apple Music, Apple News+, Apple Arcade, or Apple TV+, you don't need to cancel those subscriptions. They will automatically be canceled when you're billed for Apple One."

Apple One "In case you missed anything." FAQ at the end of this page --> https://www.apple.com/apple-one/

Same applies. Click on this link to read how to 'Get an Apple Music Family subscription" --> https://support.apple.com/HT205595

"If someone in the family group already has an individual or student subscription to Apple Music, renewal of that subscription is automatically canceled when they join the family group."

What about family sharing? Will my family member lose the music they saved from their subscription?