You may experience issues if you haven't renewed your Office 365 Alumni plan to a new plan

The Exchange Online (Plan 1) for Alumni plan has been updated. Users in your organization who are assigned to this plan must be reassigned to the new plan or risk losing access to their alumni mailboxes. Users who are still assigned to the old Exchange Online (Plan 1) for Alumni plan on December 15, 2016 may lose access to Exchange Online and be disconnected from their mailboxes. 

For more information about what happens to data and access when a subscription is disabled, see What happens to my data and access when my Office 365 for business subscription ends?

To reassign users in your organization to the new Exchange Online (Plan 1) for Alumni plan, follow these steps.

Step 1: Determine whether you already have the new license

If you're a Volume Licensing customer, the new Alumni licenses are already provisioned in your tenant. In the Microsoft 365 admin center, you'll see the old Exchange Online (Plan 1) for Alumni subscription in Expired status and the new Exchange Online (Plan 1) for Alumni subscription in Active status.  

To determine whether you already have the new license, use one of the following methods:
 

In the Office 365 portal

1. Sign in to Office 365 (https://portal.office.com), and then click Admin to open the Microsoft 365 admin center.
2. Select Billing, and then click Subscriptions.
3. Review the active subscriptions that are listed on the page.
   
   If you see two subscriptions listed as Exchange Online (Plan 1) for Alumni, you already have the new subscription.
4. Do one of the following:
   • If you have both subscriptions, go to step 3 to apply the new licenses to users.
   • If you don't have both subscriptions, acquire the new Exchange Online (Plan 1) for Alumni subscription before you to apply licenses to users. To do this, go to step 2.

In Windows PowerShell

1. Start Windows PowerShell.
2. Run the following command, and then connect by using your admin account:

   Connect-MSOLService  

3. Run the following command:

   Get-MSOLAccountSku | where {$_.accountskuid -like "*Standard*Alumni"} | fl SkuPartNumberI 

4. Examine the output. If you see both EXCHANGESTANDARD_ALUMNI (old) and EXCHANGE_STANDARD_ALUMNI (new), you already have the new subscription.
5. Do one of the following:
   • If you have both subscriptions, go to step 3 to apply the new licenses to users.
   • If you don't have both subscriptions, acquire the new Exchange Online (Plan 1) for Alumni subscription before you to apply licenses to users. To do this, go to step 2.

Step 2: Acquire the new licenses

To acquire the new licenses, follow these steps:

1. Sign in to Office 365 (https://portal.office.com), and then click Admin to open the Microsoft 365 admin center.
2. Select Billing, and then click Subscriptions.
3. Locate your existing Exchange Online (Plan 1) for Alumni subscription, and then select it.
4. Note the renewal date and the number of licenses purchased.
5. Click Add subscriptions.
6. Under Other plans, locate the Exchange Online (Plan 1) for Alumni plan, and then select it. Make sure that the plan that you select contains Exchange only.
7. Select Buy now.
8. Enter the same number of licenses that you noted earlier in step 4.
9. Clear the Automatically assign to all of your users with no licenses check box.
10. Select Check out now, and then verify and complete your $0 purchase.

Step 3: Apply the new licenses

If you have a limited number of users or for testing, you can apply the new licenses in the Microsoft 365 admin center. To do this, follow these steps:

1. Sign in to Office 365 (https://portal.office.com), and then click Admin to open the Microsoft 365 admin center.
2. Select Users, click Active users, and then select the user or users who you want to modify. 
3. Click Edit next to Product licenses or click Edit product licenses.
4. Expand the two Exchange Online (Plan 1) for Alumni licenses entries. Note that the new license doesn't include Yammer.
5. Do the following:
   1. Turn on the Exchange Online (Plan 1) for Alumni license that doesn't include Yammer.
   2. Turn off the Exchange Online (Plan 1) for Alumni license that includes Yammer.
6. Click Save.

To apply the new licenses to many users, you can use the Update-AlumniSku.ps1 script. For more information, see Update-AlumniSku.ps1.

Step 4: Remove the old licenses

After you assign the new licenses to users in your organization, remove the subscription for the old licenses. To do this, follow these steps: 

1. Sign in to Office 365 (https://portal.office.com), and then click Admin to open the Microsoft 365 admin center.
2. Select Billing, and then click Subscriptions.
3. Locate the old Exchange Online (Plan 1) for Alumni subscription by looking at the Assigned number and the renewal date that you noted earlier in step 2.4.
4. Click More actions, and then click Cancel subscription.
5. Enter your feedback, and then click Cancel subscription.