You are looking at a radical change, 10.8 Server is not an "Upgrade", it's an application. The previous Apache is still at its old location but is unused xcept for some .conf files. The arrangement and configuration of 10.8 Server.app is a big change from 10.7 so plan on some relearning. You will buy the application Server.app which on first run installs components at /Library/Server/*. Most everything is now inside the Server.app package in the Applications folder including all scripts for Wiki pages and several Apple specific Apache modules. Apple is also replacing the messy *.conf flat text files with plist files so that is an improvement. In addition, Python script support is now built-in as well as WSGI for those running Python webapps.
Having said that, some parts have sharp edges so don't expect full operation for a few versions.