As the theme uses a unique type of Featured Images function, it removes the featured images that were previously connected to the post. Given that I have over 400+ posts, it’s a lot of work to reinstate those one by one. Can you think of a way to assign the previously existing featured images to each post in bulk?
As for the featured inmages, actually the theme uses the images set in the standard WordPress fashion — in the “Featured Image” metabox. For instance, here is how the featured image for the first post in the demo (http://narrator.mauer.co/) is set in the Admin Panel:
You can see that the featured image is set in the standard metabox.
And yes, the theme uses its own function for outputting the featured images in lists (not on single post pages), but that function is just a wrapper for the standard WordPress function. It just checks if a dedicated image for post lists has been set. And if it has not, the function just calls the standard the_post_thumbnail() function:
So basically if you have your featured images attached to your posts in the regular WP way, they should display. I guess the cause of the images not being displayed is something different. Can you please email you WP Admin credentials to pavel at mauer.co so I could take a look under the bonnet?