I have not found the post if it already exists...
But I have to reconfigure a lot of UF(s) to check-in with a new DS.
Unfortunately the original DS was not configured with a FQDN.
Is there a method to send the UFs an app (e.g. "update_DS_IP) that will configure the UF to connect to the new DS (i.e. replace the DS IP)?
If you current have an app that tells the UFs where to find the DS then all you need to do is update that app with the new DS's address. Each UF will update itself and contact the new DS.
Since you are asking this question, I'll assume you do not have such an app today. You probably have $SPLUNK_HOME/etc/system/local/deploymentclient.conf on each UF. This practice is strongly (perhaps not strongly enough) discouraged for exactly this reason.
Create your app. Call it something like 'org_all_deploymentclient' and push it to all UFs.
Then comes the hard part. You need to delete the $SPLUNK_HOME/etc/system/local/deploymentclient.conf file from every UF. Let's hope you have a centralized way to do that (Puppet, Chef, etc.), otherwise you will have to log in to each UF to delete the file.
If you're brave...
Scripted input to run a bash script which rm's $SPLUNK_HOME/etc/system/local/deploymentclient.conf
Tried it once. Worked, but didn't sleep for 3 days afterwards.