Upgrading Leopard Server from 10-client to Unlimited
Success! I purchased an open-but-unused retail copy of OS X Server v10.5.4 RTL Unlim Client (product number MB605Z/A) and replaced the license key on my server from the 10-client version with the Unlimited key included in the new box. 30 seconds later I was up and running! So yes, it is simple to apply the new key and upgrade your access, and the exact version number does not need to match that which you already have installed.