It won't work. You have to have the $499 Snow Leopard Server product already installed to get the $49 dollar upgrade to Lion Server. It says this on the Apple site.
No. That page starts off with "Upgrading your
Mac server from Snow Leopard Server to Lion Server ..." [emphasis added] (
http://www.apple.com/macosx/server/how-to-buy/ )
That is a description of how to upgrade your
current server to Lion Server. There is a significant number of Mac OS X Servers out there that are
not at the 10.6 level. In fact, I'd bet it is at least 35% and wouldn't be surprised at all if it is over 50%. ( The whole "if it ain't broke don't upgrade it" syndrome). If you throw out the Mac mini servers sold over last year from the population, I would raise those percentages even higher.
What they are illustrating is that there is no path to Lion except through Snow Leopard. There isn't going to be 10.4 -> 10.7 installer. Nor will there be a 10.5 -> 10.7 installer.
Additionally if your sever has lots of user data on it LDAP data, files, users acccounts , configuration settings that took weeks to tune , etc. etc. then you probably want to do an inplace upgrade. To do a " in place " Server upgrade you need SL Server. That's is the constraint they are outlining.
In the keynote they mentioned that "Lion Server" is just a set of apps. It is not a separate OS product (no separate OS SKU, product, disk , etc.) . In other words, it is the same OS with another set of apps on top. So if you have Lion installed on your machine, you
can install Server. I don't see where there should be any doubt on that issue.
If you want to install a blank slate Lion Server you only need Lion installed and then buy/download the Server app. There is not going to be a need to go back and buy SL Server after the Macs start shipping that have Lion preinstalled on them.