gistfile1.php

<?php

// remove the standard button that shows after the download's content
remove_action( 'edd_after_download_content', 'edd_append_purchase_link' );

// our own function to output the button
function sumobi_edd_append_purchase_link( $download_id ) {
	if ( ! get_post_meta( $download_id, '_edd_hide_purchase_link', true ) ) {
		echo edd_get_purchase_link( 
			array( 
				'download_id' 	=> $download_id, 
				'class' 	=> 'edd-submit my-new-class', // add your new classes here
				'price' 	=> 0 // turn the price off
			)
		);
	}
}
// rehook/add our function back to the same location as before
add_action( 'edd_after_download_content', 'sumobi_edd_append_purchase_link' );