Mass Insert Media Attatchments into Posts

My site: 8bitnerds.com It currently has an under construction page.

My Issue

My old theme used the featured image inside the post.

My new theme does not do this. So, I have 1600 blank posts.

When I edit a single post I can insert the image because it is attached to the post.

Is there a way to mass edit all of my posts so that the image that is already attached to the post will appear in the body of the post?

I have searched for hours looking for a hack or plugin but have come up empty.

I would like to avoid editing all 1600 posts, which would take me weeks to do.