Editor's Note: Node Convert has not been ported to Drupal 8, but a replacement module called Convert Bundles is under development. At the time of writing, Convert Bundles was still in alpha testing, so it isn't ready for prime time yet, but is worth watching for future developments.
The Node Convert module can be used to switch between different page layouts without losing existing content. One example is changing a Basic page so that it uses a Horizontal Landing Page layout.
If your site is using the official GT Theme and the GT Editor module, then you should have a Styles drop-down option in the toolbar of your WYSIWYG (GUI) text editor (AKA CKEditor) with the following formatting styles. If you are not using both the official theme and the GT Editor module, then you will not see any of these styles (and may not even see the Styles drop-down option).