Hell
From NBU domain standpoint I see no difference when decommissioning NBU appliance media server or any other media server -> non-appliance one.
there is a command nbdecommission
old server must be specified using -oldserver
Usage:
nbdecommission -oldserver <hostname>
[-list_ref | -newserver <hostname> [-bulk_media_move] [-file <op_dump_file>]]
[-machinetype media | foreign_media | ndmp | replication_host]
[-M <master_server>] [-reason "<reason>"] [-v]
nbdecommission: -migrate_cloudcatalyst
[-oldserver <hostname> -oldstorageserver <storageserver> -oldstorageservertype <type> -cloudbucketname <bucket>]
[-username <username>] [-password <password>] [-kmskeygroupname <key group name>]
[-newdv <disk volume>] [-newdp <disk pool>] [-dryrun] [-start_with "<start with phrase from previous attempt>"]
nbdecommission: -delete_cloudcatalyst -oldserver <hostname> -oldstorageserver <storageserver> -oldstorageservertype <type>
Nontheless there is some documentation for that:
https://www.veritas.com/support/en_US/doc/99497890-127121011-0/v81612748-127121011
not sure what next will happen to this appliance - maybe you have to wipe its disks etc...