Pre March 2008 posts featured images aren't showing

I’ve been running a site for nearly 10 years. Upgrading, hacking and bending WordPress as need be. Before March 2008 my posts didn’t have featured images. The thumbnails were added via the excerpt bit.

I’ve run a plugin to generate featured image thumbnails from the first image in each post. It didn’t work. But what I’ve noticed is that my older posts seem to have a slightly mislinked featured image.

If I open an old post it says “Set featured image.” So I click that and there is an image attached to the post. It’s already selected as if WordPress knows it should be the featured image for the post. If I click ok and update the post the featured image now works.

What I’d like to do is run a SQL query to update all these old posts. I just can’t figure out what the query would need to be.