When you switch Office 365 for business plans automatically by using the Switch plans button, your services and billing are affected.
Access to services
Admins won't be able to use the Office 365 portal while the plan is being switched. This can take up to an hour.
Users will experience no interruption of service. They will continue to have the existing service until the switch 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 switch 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 switch plans.
If you have more than one subscription before you switch 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 switch, including Exchange mailboxes and SharePoint Online documents, lists, and other information.
The day your plan switch 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 switching to the new subscription.
Note: The length of time it takes to actually credit your payment account depends on the payment method that was used for the subscription.
Switching 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 switch until closer to your prepaid subscription’s expiration date.