Netbackup Agents: Reusing Higher tier Licenses

Hi All

I'm wondering about reusing Netbackup Enterprise Server licenses.

Specifically, If I have licensed a high tier server (say, tier 4 for the sake of argument). I then chose to retire that server, and replace it with a tier 2.

Can i reuse the tier 4 license on the tier 2 server, or are the licenses only usable on a server of the correct tier?

Thanks