Modify WP_Post before processing

There is a the_post hook which should let you modify the post object:

function my_the_post_action( $post_object ) {
    // modify post object here
}
add_action( 'the_post', 'my_the_post_action' );

But the post data isn’t where your thumbnails are stored. They are associated via a post meta field. That is what you are going to have to manipulate. Something like:

function my_thumbs($n, $object_id, $meta_key, $single ) {
  if ('_thumbnail_id' == $meta_key) {
    echo 'image link';
  }
}
add_filter( "get_post_metadata", 'my_thumbs', 10, 4 );

Though I have no idea how complex that function would need to be.

Honestly, if you are running WP 2-something you need to update and spend the time converting your improvised system to Core functions. Sincerely.