Some themes and plugins use custom post types to store data for items like portfolio pieces or testimonials . This can be an issue if you want to switch to a new theme that doesn’t have support for that post type, or your plugin is no longer maintained.
Luckily, there’s a couple ways to convert custom post types to standard post types. I recommend using Post Type Switcher if you just have a couple items to switch. For converting posts in bulk, Convert Post Types is what you’ll want.