Update Wordpress Site URLs for images and links with a plugin

Velvet Blues Update URLs — WordPress Plugins

As of 1/18/18, the plugin has a five star rating from Wordpress users, has 300,000+ active installations, and was last updated 4 days ago.

It's tested to be compatible up to Wordrpess 4.9.1

The plugin requires version 4.5 or newer, so some really old Wordpress installations may need to have the site URLs replaced en masse with PHPMyAdmin or another plugin.