I think the easiest way to do this would be to re-install NetBackup on that media server and choose the master server option. Once you have the software installed, configure your robots/drives, and perform a catalog restore
We've built our NBU masters on identical hardware just for this reason. If we needed to, we could image the original NBU master (we make these periodically) and restore it to the (in your case) NBU media server & be up and running quickly.
If you run EMM on the NBU master like we do, changing the NBU master may not be enough. EMM holds all the data of who wrote what where when, etc.
We can restore the EMM data to get the catalog up to date from there.
I am just wondering about this "promote media server"
Maybe its not as straight forward as it sounds. I thought you could have a media server running and "promote" it to a master server but then I suppose the hostname has to be the same as master server you are recovering the catalog.