Update the Device:
Remove the old SIM and add the new SIM to the relevant device.
Click in to the device via the subscription or via the user
Click on Settings in the top right corner > Manage SIMs
Under ‘Add SIM’ enter the new SIM’s ICCID
Under ‘Remove SIM’ make sure the old SIM is ticked
To save, click on Add SIM on the bottom right
Update the Subscription:
Now you can exchange SIMs as normal
Find the subscription by clicking into the Subscriptions tab > searching for the old SIM’s ICCID
Click on the Actions button on the top right > click on Exchange SIM
Enter the new SIM’s ICCID and click on Exchange
This can take a few moments to complete, please don't refresh your page
The reason we need to update the Device first is because AT&T currently requires us to submit an IMEI as well as a ICCID when exchanging SIMs. This relates to their whitelisting policies only accepting certain devices.