Hello @Lacey,
Thanks for asking. 
If you wish to switch Theme but the content was created using Cornerstone then ideally things should work fine, however please note that the bundled version of Cornerstone is tightly integrated with X Theme and upon switching theme, things may not work properly as they are supposed too. In that scenario, you will need to purchase a standalone version of Cornerstone (link shared below).
That being said, if you choose to switch Theme and page builder then those pages won’t be editable in a different page builder tool because of the structure and shortcodes. For example, content edited using Cornerstone is not editable in Visual Composer and vice versa as both are not backward compatible. However, both can co-exist at the same time. So, you can create a page using Cornerstone and other page using Visual Composer.
Thanks.