Have a look at Alex Davies' White Paper on NetBackup High Availability:
http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_implementing_highly_available_dr_with_veritas_netbackup_01_08_13599373.pdf
5 Different recovery options are discussed - one of which is promoting media server to master.
Hopefully this doc will answer all your questions.