Change Attachment Parent allows you to edit the parent value of anything in the media library.
This becomes useful if you have uploaded images you’d like to place in a post’s gallery that are not currently children of that post.
I’ve also found that sometimes you add a new post with photos and they are not added to the gallery because you hadn’t saved the post when you uploaded them. This allows you to associate those photos with the post.
The interface isn’t that user friendly, you must enter the post ID, and I’d appreciate feedback on how to improve this.
1. Extract change-post-attachment.zip and place change-post-attachment.php `/wp-content/plugins/` directory
2. Activate the plugin through the ‘Plugins’ menu in WordPress