You should also read my article below first

https://www-secure.symantec.com/connect/articles/p...

Hi

If you are upgrading on same Media server from 12.5 to BE 2012 it should be an inplace upgrade , so should be a very simple task ,but what changes you can expect for is listed in link below as BE 2012 is completely different than older version of backup exec till BE 2010 so check this link carefully and also I would recommend before upgrading it on production enviorment please have it install once on your testing enviorment and see how it works and its differences and once you are sure about all then upgrade in your production eniorment

http://www.symantec.com/docs/TECH180232

http://www.symantec.com/docs/DOC5216

Hope that helps

Thanks