The featured image upload method differs from theme to theme, as well as their sizes. So it’s natural that when changing to a theme with a different featured image method, that you’ll need to redefine / re-upload your posts featured images.

To fix this, you can rebuild all your media thumbnails, which will get your thumbnails into the new sizes specified by the theme.

If your blog / site still doesn’t show the featured images, check in your theme’s options (Appearance → Theme Options) if there’s any available option that will use the first uploaded image in the post as the featured image.

Ultimately you’ll need to edit your posts and redefine / re-upload your featured images or choose a theme with the same featured image upload method.