This has been coming up a few times in recent days on forums and distribution lists.
The question is can I install Lync Eval now, and then transfer to full Volume Licensed RTM code when it is available.
Well yes you can and it is supported.
After you get access to volume bits through a normal sources like the volume license site:
On every machine which has EVAL version of Server.MSI installed:
1. run "msiexec.exe /fvomus server.msi EVALTOFULL=1 /qb (Note: the server.msi is from Volume media)
2. run PS cmdlet "enable-cscomputer"
Hope that helps