Refresh Images attached to posts

Since years I have been creating posts and created close to thousand of posts, I have attached couple of images to posts, Now I have changed my theme and I need to refresh all my old posts to only show the images that are really in the post.

Example: Post 1 has a1 a2 and a3 attachements.
post 1 – a1,a2,a3

Now I deleted a2 and a3 from the post.
post 1 – a1

I need to refresh my post to show only a1 and show a2 and a3 as unattached in media library.