this basic step I migrate to new OS or Reinstall OS.
Take system backup
#Directadmin relate installed
/usr/local/directadmin/conf/directadmin.conf
/usr/local/directadmin/custombuild/options.conf
/usr/local/directadmin/custombuild/php_extensions.conf
/usr/local/directadmin/custombuild/custom
/usr/local/directadmin/template/custom
/usr/local/directadmin/script/custom
/usr/local/sysbk
/usr/src # If you have any custom Source script
/usr/local/src # If you have any custom Source script
###CSF Firewall ( if you have installed csf firewall )
/etc/csf
/usr/local/csf/bin/regex.custom.pm
###SSHD Config ( if you custom this file )
/etc/ssh/sshd_config
#### to make sure, you not missing any single file, just backup "/usr/local/directadmin/"
if you just install basic thing, Don't have addition custom file other than these list. this should be enought.
then create full User Backup at "Admin Backup" page
Download all backup from System backup and Admin backup
Then reinstall new OS.
so Next step just restore by "tar" tools with same path above when you take backup from "system backup"
after you restore all backup from system backup. execute install directadmin. "setup.sh".
after you installed directadmin. Then restore csf firewall at
"/etc/csf/",
"/usr/local/csf/bin/regex.custom.pm"
then restart service by
service csf restart
service lfd restart
and last thing, Restore sshd_config at "/etc/ssh/sshd_config" and run
service sshd restart
and it all ready to restore all Users at Admin Backup.
###EDIT
i add "/usr/local/sysbk" to list of system backup
sysbk will store path all list file and folder that working in "System Backup page"