Custom Field Thumbnail to new Post Thumbnail

Hi everyone,

Many of us use custom fields to attach thumbnails to our posts. With the new version of WordPress, there is now a post thumbnail feature. Is there a best practice on how to convert your current custom field thumbnails to the new 3.0+ post thumbnail?

I know that there would also be code changes involved with the theme being used. But is there a sql query we can use to convert? Or do we have to manually update each post?

Thanks!