The master server software also contains the client software, so this is why NetBackup is only allowing you to install the software on a remote system. For licensing purposes, the NetBackup Master Server software on the old master server must be removed if the same license key is being used by the new master server.
The master server is also a client in that it can back itself up.
Once the master server software is unistalled, the client software can be installed on the server.