HTTPS conversion tool
The HTTPS conversion tool in Moodle 3.4 onwards will convert any embedded HTTP content (not links) on your site to HTTPS in order for it to display correctly on your HTTPS site. It is accessed from the HTTP security screen of Site administration, or by typing directly YOURMOODLESITE.TLD/admin/tool/httpsreplace/index.php (where YOURMOODLESITE.TLD is the name of your site).
Before performing the conversion, content will be scanned to find any URLs which may not work after conversion i.e. any domains which do not appear to support HTTPS content. After switching to HTTPS, the content included from these sites will no longer display within Moodle for users with secure modern browsers. These problematic domains will be converted, but obviously won't work once the conversion is complete, hence they are reported for audit prior to the conversion process.
There is also a CLI conversion tool. To list domains that need conversion:
php admin/tool/httpsreplace/cli/url_replace.php -l
To perform the conversion:
php admin/tool/httpsreplace/cli/url_replace.php -r --confirm
To view Help:
php admin/tool/httpsreplace/cli/url_replace.php -h