Currently Being ModeratedApr 20, 2013 11:58 AM (in response to xz4gb8)
Purchase a Snow Leopard Server license from Apple, and then enter the provided license code into the license field in Server Admin. (Server Admin.app > select the server in the left column > Settings > General)
Currently Being ModeratedApr 20, 2013 12:07 PM (in response to MrHoffman)
As I tried your answer, my server popped up with the expired license notice and the opportunity to enter the new license. Since I already had purchased SLS, I entered the new licence and all was golden.
So thank you (and Apple, to some extent) for your help.