To remove a license or upgrade a station license, the phone device must be removed. All sharing must be removed. The station must be removed from all hunt group, page groups. You must remove all monitoring. You must also remove any CALL RECORD licenses from the user. Once these items are completed, you will see the Unassign option available under the Station, Device and User Assignment tab.

You must remove all these items and unassign the license to upgrade a license.