Upgrade Snow Leopard Server to Lion Server. What problems should I expect to arise? Is upgrading as simple as installing Lion Server from the App store? I've heard that you have to be running Lion first to be able to install the Lion Server software, but I'm looking at upgrading a live server and need to know how others have gone about it and the problems they ran into. Thanks :0)
Well, I would advice clearly against doing this for a live system at the moment. Don't fix it if it isn't broken.

Lion Server has a completely revamped UI, it takes quite a bit of time to reconnect all the bits and pieces. It does a lot under the hood which used to be exposed in Server Admin.
DNS / DHCP / other low level services are still available, but not exposed in the default UI anymore.

Set aside a separate machine, learn to find your way in the new UI, and then you may choose to upgrade your live system or not.