Upgrade database URL loops to WP login screen but works on a cloned staging site
I stumbled across a work around purely by accident but to cut a long story short:
Problem is that since the upgrade database url points to an unsecure http file, my server then wants to redirect to the secure url - resetting the session - requiring an admin login (not sure why this is since automatic SSL redirects have been disabled). Using the upgrade url, I deliberately added an additional character/integer to the url end, forcing the browser to navigate to a http page that doesn't exist. Now that I was sitting in a unsecure 404 page, I simply deleted the previously added character/integer and voila; no redirect to a secure page and consequently the upgrade screen correctly loads.