Because we are preparing to roll out Windows 10 in our enterprise, I am working to decommission the old Server 2008-based KMS server and am loading up a new one based on Server 2012 R2 Standard.

We have multiple domains (and more than one forest) in our environment, and we have used KMS quite effectively by simply adding _VLMCS records for to the IP address of the KMS Server into DNS in each domain and verifying the traffic could reach the server through our routers and firewalls.  

Because of the complexity of the network, I plan to assign the same IP address to the replacement server, and I want to get it loaded up and activated before connecting it to the network and joining it to the domain.  Does anyone foresee any issues with our loading and activating the server (with the KMS key) via telephone, patching the server as a non-domain member, and then changing the IP address and joining the domain as the last step?

Thanks,
Steve

That should work out fine, just keep your eye on the DNS SRV RR's in the server's own DNS namespace, to make sure that it registers/publishes the expected hostname and IP into the RR's.

Domain membership itself isn't required/relevant, unless that's how you deliver DNS suffix and DDNS registration permissions..