What does upgrading Office 365 plans do to my service and billing?
When you upgrade plans automatically by using the Switch plans button (or the Upgrade tab in the new admin center), your services and billing are affected.
Access to services
Admins won't be able to use the admin center while the plan is being upgraded. This can take up to an hour.
Users will experience no interruption of service. They will continue to have the existing service until the upgrade is fully completed.
Users and licenses
Users on the old subscription will automatically be moved to the new subscription.
If your old subscription includes multiple services, and if you have changed which of these services your users are assigned to, you may want to make note of this before you upgrade plans so that you can recreate these changes afterwards. All users will be given access to all services in the new subscription. For example, if you previously purchased Office 365 Enterprise E3 for all 100 of your users, but unassigned the SharePoint Online service from 50 of them, this change would not be kept after you upgrade plans.
If you have more than one subscription before you upgrade plans, and have users assigned licenses to more than one subscription, this assignment pattern will be kept as much as possible in the new subscription.
All user data will be retained during the upgrade, including Exchange mailboxes and SharePoint Online documents, lists, and other information.
The day your plan upgrade is complete, the billing on your old subscription will be turned off and the billing on your new subscription will be turned on. You'll get a prorated credit for any unused service on the old subscription. You'll receive a new invoice that includes the credit for your old subscription within 30 days of upgrading to the new subscription.
The length of time it takes to actually credit your payment account depends on the payment method that was used for the subscription.
Upgrading from a prepaid subscription before it expires? If the total cost of your new subscription is greater than or equal to the remaining value of your prepaid subscription, you won't lose any prepaid time. On the checkout page, you'll see a credit for your unused time. However, if the total cost of your new subscription is less than the remaining value of your current prepaid subscription, you'll forfeit some of your unused time. You'll be notified before you checkout, and you can wait to upgrade until closer to your prepaid subscription's expiration date.