How to let contributors to create a new revision(draft) editing their published posts

Using Role editor or role scope you can set contributors to edit you custom post type but not publish so every change will be set as draft until approval, and to limit the creation of new posts of your custom post type you can use my plugin Bainternet Posts Creation Limits

Update

To force re approval of edits add this code

add_filter( 'wp_insert_post_data', 're_aprove', '99', 2 );
function re_aprove( $data, $postarr ) {
    //check if current user is not admin
    if ( ! current_user_can( 'manage_options' ) && 'YOUR_CUSTOM_TYPE' === $postarr['post_type'] ) {
        if ( 'publish' === $data['post_status'] ) {
            $data['post_status'] = 'pending';
        }
    }
    return $data;
}

and change YOUR_CUSTOM_TYPE to your custom post type name.