The ONLY supported solution is to get Symantec Consulting to do the change...
http://seer.entsupport.symantec.com/docs/268696.htm
Note: Do not change the host name of a NetBackup server. This practice is not recommended because it can be necessary to import all previously used media to the server before you can use it under the new host name.
Changing or modifying any NetBackup server host names must be done with extreme care, whether they be Master Servers or Media Servers.
It is highly recommended that any NetBackup Server host name changes be done through Symantec's consulting services. This is due to issues such as the server host name being appended to the images in the catalog. Simply changing the Master Server host name can cause restores to fail, connection problems with clients and having to import all media back into the catalog.
Changing or modifying the master host name is not supported by Technical Support without the assistance of a qualified Symantec Consultant on site.