By setting every possible setting available within the gitlab.rb I was able to perform the 8.4 update and still log back into Mattermost.
After this, it only lost some settings related to webhooks (enable incoming and enabling overriding usernames and icons) which is a very minimal action to perform after upgrades (and which may be prevented by copying/replacing the Mattermost config.json file).
So, that was pretty much the ticket as the big issue was losing the auth/access. Thanks much!
tpro