When trying to upgrade from one version of BE to another (in this case, from 12 to 12.5) you may receive an error windows that starts:
"The Backup Exec database version you are trying to use…"
This is a complete red herring; I spent ages uninstalling SQL 2005 express, re-installing BE, uninstalling BE & SQL 2005 then deleting SQL 2005 folders to no avail. This article:
pretty much says it all; I would uninstall anything Symantec related, uninstall SQL 2005 then remove any SQL/ Symantec folders under %PROGRAMFILES%… then try again (which is what I’m about to do).