You can not "promote" a media server into a master server.  You'd have to decommission the media server - and there are rules to doing this (don't forget the media servers own the tapes!) - and then deploy it as a master.  You could certainly do this and as part of your upgrade project, re-image your old master server and re-deploy it as a media server.  Just follow all the steps to do so.  Make sure the master server does not change names during your upgrades.

You also need to be aware of the licensing implications.  Media servers are not cheap and you would also need SSO licenses to share your tape drives.  I don't know how the VTL licensing works these days.  That might vote against using the new server now.

Personally, I'd start working on that new server with the intention of making it the master.  That gives you a lot more flexibility for a rollback when you do the upgrade - bring up NetBackup on the new server, and if it has issues, you can go back.  If you've already wiped it as a media server, you've got a significantly more complicated rollback since now you're down a media server as well.