DSmidge

When WP is opened through HTTPS, all local absolute links in content are replaced from HTTP to HTTPS. 
Additional hosts can be specified through admin interface (like graph.facebook.com which works on HTTP and on HTTPS). 
Options HOME and SITEURL shuld be changed to HTTPS when enabling this plugin.

HTTP to HTTPS link changer by Eyga.net

When WP is opened through HTTPS, all local absolute links in content are replaced from HTTP to HTTPS.

JS Morisset

A simple, safe, and reliable way to force HTTP URLs to HTTPS dynamically:
No setup required – simply activate the plugin to force HTTP URLs to HTTPS.
There are no plugin settings to adjust, and no changes are made to your WordPress configuration.
SIGNIFICANTLY FASTER than other popular plugins of this type:
Other well known plugins use <a href="https://secure.php.net/manual/en/function.ob-start.php" rel="nofollow ugc">PHP’s output buffer</a> to search & replace URLs in the rendered HTML, which is a technique that is error prone and negatively affects caching performance (as changes are not cached).
This plugin uses standard WordPress filters instead of PHP’s output buffer for maximum reliability, performance, caching compatibility, and uses 301 permanent redirects for best SEO results (<a href="https://en.wikipedia.org/wiki/HTTP_301" rel="nofollow ugc">301 redirects are considered best for SEO when moving from HTTP to HTTPS</a>).
Supports advanced proxy / load-balancing HTTP headers:
<ul>
<li><code>X-Forwarded-Proto</code> (aka <code>HTTP_X_FORWARDED_PROTO</code> server value)</li>
<li><code>X-Forwarded-Ssl</code> (aka <code>HTTP_X_FORWARDED_SSL</code> server value)</li>
</ul>
See <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto" rel="nofollow ugc">Web technology for developers > HTTP > HTTP headers > X-Forwarded-Proto</a> for more details.
<h4>Plugin Requirements</h4>
Your web server must already be configured with an SSL certificate and able to handle HTTPS connections. 😉

JSM Force HTTP to HTTPS / SSL – No Setup, Fast and Reliable

No setup required - simply activate to force HTTP URLs to HTTPS using native WordPress filters and permanent redirects for best SEO.

HTTP to HTTPS link changer by Eyga.net vs JSM Force HTTP to HTTPS / SSL – No Setup, Fast and Reliable