How to rename a list of clients that do not have the correct name
I am afraid of making a new mistake so I come to ask for help here so I can rename all NBU clients correctly according to their machine name.
You cannot directly execute custom code via NetBackup Client. (well, that's not strictly true, as you can via pre and post scripts surounding backup jobs - but that might not work in a moderm certificated environment with clients that have an incorrect internal name). And, you might be able to change/update the client name of all the clients from the Master using the bpsetconfig or nbsetconfig commands, again as long as certificated authentication does not get in the way (because the internal client names are wrong) - but you would need to know the intended client names beforehand.
Instead, I would think it would be a lot easier if you got SCCM to execute some custom code to change the "client_name" NetBackup Client registry entry... or wrote your own custom PowerShell to do the same.