When you change your theme but will be using same content and images, you would need to regenerate the featured images, so that they match the new sizes specified in the new WordPress theme.
To do this you need to use an image regenerator plugin. We suggest using this one https://wordpress.org/plugins/regenerate-thumbnails/
After you install and activate the plugin, in your Dashboard go to Tools → Regenerate Thumbnails. Click on the button to Regenerate, and let the plugin do the rest.