So I successfully performed this update last night. Elapsed time - about 4 hrs. (most of that time was waiting for the Mavericks download and the pre-treatment stuff I outlined. Use at your own risk - no guarantees of course but overall I was delighted with the ease of the process.
1) Check Compatibilities
2) Rebuild permissions on target server - target disk - from the recovery disk.
3) Run any/all patches/updates.
4) If upgrading an OD Master follow these procedures: http://support.apple.com/kb/HT5300
5) Post updates permissions rebuild again on target server - target disk - from the recovery disk.
6) Backup via Time Machine - and a "bare metal" copy program (i.e. SuperDuper or Carbon Copy Cloner) to an external or separate internal disk.
7) Test the backups (the bare metal at least) - and not just successful boot. Test all services functionality, network configurations, web functions etc. Note: SuperDuper's performance was flawless.
8) OPTIONAL: Run all Apple updates and rebuild permissions on target server - MIRRORED DISK - from the recovery disk.
9) Boot back to your Target Disk
10) Stop all Server services/functions. I did NOT drop the server from the AD domain (though I DO do that for Lion to Mavericks WORKSTATION upgrades now)
11) Make sure that any DNS or DHCP servers that your server relies on remain running during the process. (Source)
12) Upgrade the OS on the target machine to the latest version of OS Mavericks via the App Store (making sure not to launch any legacy Server Apps afterwards - their icons were deprecated on the dock anyway)
GOTCHA ALERT: Be sure you have an Apple ID tied to a credit card setup on the machine you're updating. Otherwise you'll be asked to enter the CC security ID on the different machine. I was able to get around not having this available at the time of the update by going to the machine I enrolled the credit card under and buying/downloading OS X Server. Then I copied the installer to the target server - worked fine.
13) Check for any updates needed through the App Store. If any are needed - run those updates and again, reboot/rebuild permissions from the recovery disk.
14) Upgrade the Server App to OS X Server from the App Store
15) Reboot
16) Rebuild permissions on target server - target disk - from the recovery disk.
17) Turn SSL back on per http://support.apple.com/kb/HT5300 if applicable. (I forgot to do this).
18) Resume Core Services
*Acccording to Apple: "In most cases, your service data and settings will be preserved completely when upgrading or migrating." This proved to be true in my case.
** Watch for the deprecated ipfw firewall surprise at the end. Even though I tested SMB connectivity before leaving, users weren't able to log in next morning. I shut down the reported "legacy" ipfw firewall which was an option listed in the log as a first suspect and it worked. Firewall settings are retained (I determined by running a port scan) but see:
http://support.apple.com/kb/ht5413
http://support.apple.com/kb/HT200259
http://support.apple.com/kb/PH13892
Potential 3rd Party Firewall GUI for Mavericks as well - check compatibility: http://www.hanynet.com/icefloor/index.html