Change permalink structure only for future posts

Hi, I am trying to change the permalink structure of blog posts only for the future posts without changing the old URLs due to their SEO scores.

When i try to change the permalink as http://www.domain.com/blog/catname/postname it works:

// rewriting post url
function format_post_url( $post_link, $post, $leavename ) {
	 if ( 'post' == get_post_type($post) && 'publish' == $post->post_status && strtotime('2017-04-01 00:00:00') < strtotime($post->post_date) ) { 

		$post_link = rtrim($post_link,"/");

		$post_pos = strrpos($post_link,"/");
		$post = substr($post_link, $post_pos+1); // post name
		
		$post_link = substr($post_link, 0,strlen($post_link)-strlen($post)-1);

		$cat_pos = strpos($post_link,"/",8);
		$cat = substr($post_link, $cat_pos+1); // category name

		if(substr($cat,0,3)=="en/"){
			$cat = substr($cat, 3);
		}

		$post_link = substr($post_link, 0,strlen($post_link)-strlen($cat)-1);

		return $post_link."/blog/".$cat."/".$post."/";

	}
	return $post_link;
}

add_filter( 'post_link', 'format_post_url', 100, 3 );

But when i try to change the permalink as http://www.domain.com/postname it gives a 404 error, like following:

return $post_link."/".$post."/";

Please note that i can achieve this easily by changing the permalink structure via the WP backend but it will alter all the URLs, past & future, but i just want to change the future ones.

Can you guys please help me ? I even update the permalink structure settings in the wp backend after making these changes but it still doesn’t work.