Apple describes three scenarios if you have an existing iCloud+ plan in What happens to your iCloud storage when you sign up for Apple One - Apple Support and there if further information to read in the linked document.
If the iCloud storage included in Apple One is more than your current plan
Your current iCloud+ plan is canceled and you'll receive a pro-rated refund. Your total iCloud storage will be the amount included in your Apple One subscription.
If the iCloud storage included in Apple One is equal to your current plan
During the free trial, you keep both your current iCloud+ plan and the iCloud storage in Apple One. Once the trial is finished, your current iCloud+ plan is canceled. Your total iCloud storage will be the amount included in your Apple One subscription.
If the iCloud storage included in Apple One is less than your current plan
You keep both your current iCloud+ plan and the iCloud storage in Apple One.
If you prefer to keep only the storage included in your Apple One subscription, you can downgrade or cancel your current iCloud+ plan.