iCloud Storage / Changing Monthly Payment Dates

I think downgrading would not work:

Downgrade your iCloud storage - On an iOS device, a Mac, or a PC. - https://support.apple.com/HT207594

"When you downgrade your iCloud storage plan, the storage decrease takes effect after your current monthly or annual subscription ends."

So you'd still be paying on the same day.

Upgrading is made effective immediately since people would not want to have to wait up to 30 days if they are desperate for space.

Buy more iCloud storage - https://support.apple.com/HT201318

"If you upgrade from one paid plan to another, we’ll cancel your existing plan and charge you the prorated cost* of your new, larger plan."

I do not see details but I am assuming this resets the plan but I guess they could just be charging you the higher rate for whatever remains until your old renewal period. In that case you will have to completely back up your files, cancel your subscription, then start it again when you want to.

You could, or course, then downgrade the next month if you do not want the higher storage.

You will have to cancel your current plan and start a new one, or maybe (I don't know) upgrading to a different plan may become immediately effective?

Here's contact information for Apple but I have never heard of them changing a billing date:

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions -->

https://www.apple.com/emea/support/itunes/contact.html

Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232

It's not clear what is pro-rated and what is not.

Scenario 1: You subscribed on the first. You upgrade on the 15th. Apple refunds (probably credits) your remaining low rate for the 16th-31st, then starts the higher rate from the 16th, billing you for a month so your plan now renews on the 16th.

Scenario 2: You subscribed on the first. You upgrade on the 15th. Apple refunds your remaining for the 16th-31st, then starts the higher rate from the 16th but only charges you enough to cover until the last day of the month and then continues at the higher rate from the first of next month.

You will have to call Apple and ask them which scenario they follow. If you find out, please report back here so we can learn something too.

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions -->

https://www.apple.com/emea/support/itunes/contact.html

Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232

The billing date is based on the date of the month you started the subscription, and it cannot be changed. As Limnos mentioned, all you could do is downgrade first, then re-subscribe on the date you wish to billed on. But make sure to backup everything somewhere else first as when you downgrade, if too much time passes before re-subscribing, you will lose data.

Downgrading would be fine if you wait until that last payment is charged and the subscription ends. Then start a new subscription on a new date. But you’d have the issue of possibly losing data if you’ve not archived it somewhere first or you allow a big lapse between canceling and then starting a subscription.

I don’t know if upgrading works given the pro-rated fee aspect.