omd update just “switches” a site from one installed “OMD” version to another - as shown by omd versions. The destination “OMD” version can also be a Checkmk version since they still use the same packaging system like in the “old OMD days” – even the /usr/bin/omd command is still there. So you won’t “lose” Checkmk if you “upgrade” to a Checkmk version. 
You can install Checkmk 1.4.x alongside your current OMD version. You should then see them both in the omd versions output and can upgrade your site in-place (instead of using backup/restore into a new empty site). Having a backup is nevertheless highly recommended, of course 
And follow the steps in the link given by marco (upgrade from one major release to the next, do not skip versions).