From the linked article:
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.
And:
Apple One includes a free 1-month trial for services that you don’t already have. After your free trial, services that you previously subscribed to are automatically included in Apple One. If you recently canceled a service, you might not be eligible for the free trial. Your plan automatically renews after the trial.
You may get one month free, then start paying the Apple One price for your selected bundle. If you have an unexpired portion of a trial or an annual payment then you may get some additional discount for the Apple One subscription until that expires, but check with Apple Support if you're unsure.
Reminds me I should really upgrade to Apple One, I've currently got Apple Music, TV+ and Arcade so it ought to save me £5pcm. Hmm, no free trial for me but I've got Apple Arcade until April so I'm spending about the same right now. Guess I'll wait until then, although I might still save a couple of pennies if I start now.
tt2