Netbackup does not use login ID and passwd for backing up and restoring clients. Instead it uses TCP/IP port to connect. The client needs to have its bpcd daemon in a listen mode. you can check that on the client with netstat. The client also has to have the master and media servers in the bp.conf file so that the client can recognize it as a valid server to use the TCP/IP port. So go ahead and change your root passwd. Backups will not fail from the changing of the passwd.