Telling FB to use a different URL to fetch shares/likes

I updated URLs for SEO purposes and had to create a custom field to store the old URL in which I could use later as the URL I wanted FB to recognize so that my FB likes wouldn’t disappear.

In the FB code, I can simply put:

<div class="fb-like" data-href="<?php sharingurl(); ?>" data-send="false" data-layout="button_count" data-width="90" data-show-faces="false"></div>

where sharingurl() is a function* in my functions.php:

function sharingurl () {
	if(get_field('old_url') == "")
		{
			echo untrailingslashit(get_permalink());
		} else {
			echo get_field('old_url');
		}
}

* I was already using the Advanced Custom Fields plugin so I used it here to create the custom field “old_url”.

How can I change your code to accommodate this? Any hints?

http://wordpress.org/plugins/bj-facebook-popular-posts/