Doing Lion evaluation and testing I upgraded a Snow Leopard drive (an external clone of a working system drive) to Lion, BUT I GOOFED and somehow upgraded to Lion Server! It attempted to upgrade my server settings, which of course were not present, but did see the existing server and integrated itself into Open Directory it provided.
My conclusion from this mini-disaster is that unlike other OS X server versions, this one will be capable of an upgrade installation. That is great news! I'm still concerned about the apparently missing services, but I think we all need to wait for Snow Leopard Server BETAs instead of just relying on this Developer Preview. At that time I'll also be trying an upgrade of Snow Leopard Server cloned on an external drives.
(FWIW, I don't intend on running any of these Lion pre-releases on any internal drive because I don't have a system that I can risk.)