Upgrading Leopard Server from 10-client to Unlimited I need the lowest-impact way to allow more than 10 AFP sessions on my OS X Server running 10.5.8. If I buy a boxed retail copy of 10.5.4 Server Unlimited, will I just have to enter the new license key in Server Settings? Is it OK if the version numbers don't match up exactly on this type of upgrade?

The specific product I'm looking at is here on Amazon. Apple product ID MB605Z/A.
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.