upgrade from dns only to full whm/cpanel without disrupting service - how?

Quick question: I currently have a vps with cpanel dns only runninng my secondary dns clustered with my main cpanel server.

I now want to upgrade the dns only to a full version of whm/cpanel on the vps and cluster both full versions.

However, I believe I need to install the full version of whm/cpanel before getting the license file, so how should I best do this without causing any disruption in current dns function?