Wordpress - Switching MultiSite installation from HTTP to HTTPS

I encountered the same issue: with WordPress multisite, there is no option or setting that defines if a site domain is HTTP or HTTPS. Even after replacing all occurrences in the database, a site visitor can still enter and navigate the site in HTTP, without being redirected to HTTPS.

The following simple solution worked for me: I added this mod_rewrite rule to my .htaccess file, just before the WordPress specific rewrite rules.

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Since this rule isn't defining a specific domain, it's perfect for the needs of WordPress multisite.

I found this rule in the following thread: https://stackoverflow.com/questions/4398951/force-ssl-https-using-htaccess-and-mod-rewrite

There are plugins to set the site to SSL. Why not use one of them?

And you do have an SSL certificate installed and active?