You’ve got Open Directory running and humming beautifully in macOS Server 5.4 (running on macOS High Sierra). You show up to work and the hard drive has died on that perfectly configured Open Directory Master. Luckily, you have a replica and you have an archive of your Master. You can restore or you can promote your Replica to a Master. What to do? Well, I can’t tell you what you should do, but I can tell you that Apple has planned for this. Here, we’re going to look at promoting that Replica to a Master. Because after all, hard drives fail. Let’s look at what all this looks like.
Create An Open Directory Archive
In order to properly restore an Open Directory Master or promote a Replica to a Master, you’ll need the SSL keys. You should also just keep archives of your Open Directory environment around (albeit in a secure location) because you really never know. To create an Open Directory Archive, which has the keys in it as well as data needed to restore a Master, first open the Server app. From within the Server app, click on the Open Directory service. 
Towards the bottom of the screen, click on the cog wheel icon. 
At the menu, click Archive Open Directory Master… 
When prompted, provide the username and password to the Open Directory environment shown in the Server field and then click on the Connect button.
At the Archive Open Directory Master screen, choose a location to create your archive. Also, provide a password for the archive. Click the Archive button when you’re ready to proceed.
At the Confirm Settings screen, click Archive. The archive is then created. Keep this safe as it has all your base are belong to us in it. You have to do this proactively. Once the hard drive in that Open Directory Master craps out, you’ll need the Archive to put the pieces of Humpty Dumpty back together again.
Promote A Replica To A Master
Provided you have a Replica and an Archive, promoting a Replica to a Master couldn’t be easier in macOS Server. To do so, open the Server app from the Replica and then use the cog wheel icon to bring up the menu. 
Here, click Promote Replica to Master.
At the “Promote Open Directory replica to master” screen, provide an Open Directory username and password (e.g. diradmin with the appropriate password). Also, choose the archive you created previously. Then click Next. 
Click Promote. 
The Replica will become an archive. Once finished, remove any other replicas and repromote them.
Stop Open Directory
Another option is to stop Open Directory on the replicas until you can get your Master back up and running. To stop Open Directory, open the Server app and click on the Open Directory service. Click on the OFF button. You’ll then be prompted to verify that you really want to stop directory services on the server. Click OK (which should probably read a bit more ominous, like “OMG, OK”. The server is then stopped. To completely remove Open Directory from the old server, run the slapconfig command, followed by -destroyldapserver:
slapconfig -destroyldapserver
Also, don’t forget to go to the Master and remove any servers from there as well, once they’ve been fully demoted. View the logs using cat for any other weirdness:
cat /Library/Logs/slapconfig.log