running update_clients, found a few issues :
1 - the script does NOT check for tmp space - if you do not have 4G space available in /tmp on the clients the install can fail - and leave the client messed up - I had to uninstall packages and delete the netbackup directory!
2 - sometimes the certificates get confused - if your client does not reconnect - vnetd error - you should run command "nbcertcmd -hostselfcheck" - make sure it is not revoked.
3 - you may have to run command "/usr/openv/netbackup/bin/nbcertcmd -getCertificate -host CLIENTNAME -server MASTERNAME" to re-establish connectivity.
I have a linux box, it would not connect, vnetd issue, but all certificates showed ok, even after the above commands.
Found out undocumented command 3 option -force - forces update of client certificate.
"/usr/openv/netbackup/bin/nbcertcmd -getCertificate -host CLIENTNAME -server MASTERNAME -force"
make a note of it!