There's this doc on Apple.com:
http://support.apple.com/kb/HT5381
It says this, rather oblique, statement:
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DHCP
The DHCP service is no longer managed as a separate service in OS X Server. DHCP is used by the NetInstall service to provide startup information to NetInstall and NetBoot clients. It can also used by Internet Sharing (in the Sharing pane of System Preferences) to provide addresses to clients sharing the server's internet connection. Only one of these two services can be used at one time.
If you enabled NAT and DHCP in Server Admin in Lion Server, your configuration will not be preserved when you upgrade to OS X Server (Mountain Lion). You can restore the default NAT and DHCP configuration by disabling then enabling Internet Sharing in System Preferences.
If you enabled DHCP (only) in Lion Server, or you enabled DHCP in Mac OS X Server v10.6, your configuration will be preserved and remain active when you upgrade to OS X Server (Mountain Lion).
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Also notice that the Firewall 'service' is gone. They are also deprecating ipfw. So your firewall rules that you already setup remain in place after the upgrade, but you have to use the command-line to edit them:
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Firewall
The Firewall service is no longer managed by OS X Server. Existing Firewall rules remain active after upgrade, but any further firewall configuration must be performed on the command line. For more information, see this article.
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I trolled around the internet, and found this useful sounding tool:
http://www.hanynet.com/icefloor/index.html
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So the question really becomes, with all the manual work now required to 'upgrade' to ML Server, why not just go with a Linux Server and do it all manually? The utility of OS X Server has become marginalized.