Normally when you upgrade a Backup Exec installation, upgrades where the major version number does not change do not require a new license. Backup Exec 12.5 is an exception to this rule.
If you already have Backup Exec 12, and wish to upgrade to Backup Exec 12.5, then you need a new license key. If you have an active maintenance contract this upgrade is included, and you can generate a new license key using the Symantec licensing portal.
License keys from version 12 and earlier will not work in version 12.5