Currently Being ModeratedJul 21, 2011 4:56 PM (in response to dlmcshan)
My understanding is that the installer will detect SL Server and automatically install the Lion Server add-on, prompting you to purchase and download the add-on. If you have a non-server Mac Mini, then you should use your original disks to reinstall the non-server SL, before installing Lion.
If you have a Mac Mini Server, then there's no supported way to install the non-server SL. You'd have to install the non-server SL somehow first, or Google one the ways to do a "clean" install of Lion. Either way, you likely will have to erase your existing SL Server installation in order to install the non-server Lion.