Subscribing to Apple One

There is a specific support article for what happens to your current iCloud plan when signing up for Apple One:

What happens to your iCloud storage when you sign up for Apple One - Apple Support

For Apple TV and Apple Music, you will receive a refund for the remainder of the current subscription in a couple of days. Do not cancel your Apple TV and Apple Music subscriptions manually, as this will only complicate matters.

You only “pay twice” temporarily.

A refund will be issues once the billing cycles are all “sync’d.”

DON’T attempt to cancel any of your old subscriptions; data loss may occur.

Let “The System” work and sort it out.