I just upgraded to the latest version of the Arras theme and my post thumbnails no longer display. For many reasons, I have continued using the "thumb" custom field for my post thumbnails, primarily in the event that I want to update a thumbnail that effects many pages, I only have to upload one picture instead of having to change many posts using the same thumbnail.

Is this method now obsolete?? If so, is there a way to make a quick change or do I need to manually update page thumbnails on over 700 posts??