Copying post thumbnail to custom field

I wouldn’t bother doing that.

I’d just find the places in your theme files that call the custom field (probably something like “echo get_post_meta(‘bigimage’)” or something) and replace those with a the_post_thumbnail() tag.

This saves you the trouble of messing with the data, and it makes administration much, much simpler going forward–which is kind of the point of post thumbnails. And it’ll probably take less time than rigging some SQL to copy all that data.