I just wanted to share with you what I have found useful for upgrading your old version of BE for Windows Servers. There are a couple of steps that must be performed before doing any upgrades in order to have sufficient backup of your current BE server. Before beginning the upgrade, you need to backup your C:\Program Files\Symantec\Backup Exec\Data; C:\Program Files\Symantec\Backup Exec\Catalogs folders (if you are using external SQL database make sure you have your BEDB backed up) but before that make sure you have stopped your BE and embedded DB service (Backup Exec Server and BEDB service), so if something goes wrong with the upgrade you'll have your Catalogs and DB with you. The following is step by step upgrade from older than 11d version ov BE (only 11d can be upgraded to 12.5) that you need to perform one by one.
Download link for the Backup Exec 11d:
Run the Browser.exe from the BE 11d folder to upgrade from 10d to 11d.
Download Backup Exec 12.5.
Run the Browser.exe from the BE 12.5 folder to upgrade from 11d to 12.5.
(Refer: Document for Multi-step upgrade path from Backup Exec 10.x for Windows Servers to Backup Exec 12.5:
Upgrade process for the Remote Agents on the Remote servers:
Uninstall the "Remote agent for the Windows Servers" version 10d from each remote servers Add/remove program.
Reboot the Remote server.
Install the Remote Agent version 12.5. http://support.veritas.com/docs/253058