Publishing the same map "replaces" the currently published map as long as the publishID inside the map file is identical and you're logged in with the same user that published the earlier version, but you can roll back via the workshop to an older version if needed. So there a rudimentary form of version control present.
The current version should be updated / downloaded after publishing automatically, will show up as GWYF Workshop Content in the Steam download tab. If it doesn't start up right away, un/resubscribing should force a fresh download.

Ok, thanks. I like to test my maps at several points to head off any obvious problems.