Change Featured Image

Hi,

I have a custom post type called stories. Basically I set the featured image as normal and it displays it as required.

However I want to add a checkbox somewhere within the post editor so when I check mark whichever post I am editing it get marked as solved and changes the featured image (already set before the re-edit) to a predesigned watermarked image replacing the original featured image.

Thus creating the posts lists with the ones not marked solved with the normal featured images and the ones marked solved with the watermarked featured images set on them.

Any help please,

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