I guess this field is grey and not allowing me to change it because the main site is already secured via my host?
It is greyed out by default by WordPress, not your host. That’s because changing the main Multisite domain requires changes at several places, and if not done properly, can break your entire multisite network (not just the primary site).
It shows a padlock when logged out on the front end of the main site only.
If you have the padlock already, it means your site already uses HTTPS… and I don’t understand why you want to switch to HTTPS again, which is why I asked to see your URL to know what’s really going on.
Anyway, here’s WPEengine guide on changing a Multisite installation’s primary domain. For switching from http to https, I believe changing the URL references in WP_OPTIONS and WP_SITEMETA database tables should be enough for the primary site, and WP_#_OPTIONS for each of your subsites.
https://wpengine.com/support/how-to-change-a-multi-site-primary-domain/