To update a DSfW server to the latest Domain and Forest Functional Levels run the following commands:
Domain: Run / Re-run the script /opt/novell/xad/sbin/ domainUpgrade.pl -D on the Primary Domain Controller of each domain.
Forest: Run / Re-run the script /opt/novell/xad/sbin/ domainUpgrade.pl -F on the Primary Domain Controller of Forest Root Domain.
For more information, see sections 9.4 and 9.5 of the DSFW Documentation for OES2018SP3, found here:
NOTE: While it states 2012, the fully update functional level supports 2016 and 2019 (since they're the same).