How to transition from multiple subscriptions to Apple One Family Premier?

I have Family share subscriptions for iCloud & Apple News paid monthly plus an individual subscription to Apple Music paid annually with about 9 months to run. I want to change all that to a Family Premier One subscription. I understand I can cancel the existing monthly subscriptions but I can't find out what happens when I cancel the annually paid sub. Does it cancel automatically when the Premier One starts, do I get an automatic prorata refund or do I have to claim it? Am I eligible to take the 'Free for a month trial' & if so it appears that I wouldn't get the things that I currently subscribe to so wouldn't be able to share the Music. Is that correct? I would appreciate a simple straight forward explanation of how to do what I want. Thanks

[Re-Titled by Moderator]

Original Title: Changing subscriptions