I figured OS X Server 10.8 was probably ready enough for me to upgrade to from 10.7, given it's half a year old and had two point releases, so did this over the weekend. This turns out to have been harder than it could have, due to the opacity of some of the components.
The App Store refused to replace Server.app from Lion Server. Fine, move aside the old one and place the new one in /Applications. This is required now because it contains many server binaries, it's not just a configuration tool.
Once this was done, the new Server.app refused to communicate with the server daemon, complaining about “SSL error has ocurred… blah blah whatever”. Solution from apple communities: Delete from the system keychain any certificates and private keys for previous computer names.
Finally, the mail server was running but broken in a few different ways. I have some custom filtering rules, HELO checks etc. The new Server.app kindly moved everything from /etc to a new home somewhere in /Library/Server/Mail/… which is apparently the cool place now. Unfortunately all the config entries in main.cf didn't know about that and were looking into /etc/postfix. OK fix all them up.
The last gotcha was the email virtual host configuration got destroyed so I had to reset them in the new Server.app.
One more thing: the server-side Address book and Calendars were destroyed. One can restore from Time Machine the address book entries by enabling debug options, but the calendar I'm going to assume is gone for good.
EDIT: Another one more thing: web virtual hosts had their locations defaulted from their custom configuration in Lion Server.
Apart from that it all seems OK now.