How can I migrate all of my custom field thumbnails to the built-in post featured image?

http://justintadlock.com/archives/2009/11/16/everything-you-need-to-know-about-wordpress-2-9s-post-image-feature

LIsts a plugin:
http://wordpress.org/extend/plugins/get-the-image

I’m not sure by the article and description if what you are looking for is exactly what it does? It seems to imply that, but I’m not 100%