Why does it matter? Just push the new client out as noted by Richard. This will both upgrade them and point them to the new site.
> "or do I have to upgrade from one version to the next until I have SCCM 1802 current branch installed."
Not following this statement. 1802 is a baseline version that your new site will be directly installed with. There are no other versions that you need although you should upgrade to 1806 immediately.
Jason | https://home.configmgrftw.com | @jasonsandys