Code snippet for update script to use new theme logo

custom-logo-update-script.php

<?php
/**
 * Theme Update Script
 *
 * Runs if the logo has not been set yet.
 */
function prefix_update_check() {

	// Return if update has already been run
	if ( -1 == get_theme_mod( 'custom_logo', -1 ) ) {
		return;
	}
	
	// If we're not on 3.5 yet, exit now
	if ( ! function_exists( 'the_custom_logo' ) ) {
		return;
	}

	// Set a default value of false for the custom logo
	$custom_logo = false;

	// If a logo has been set previously, update to use logo feature introduced in WordPress 4.5
	if ( get_theme_mod( 'logo', false ) ) {

		// Since previous logo was stored a URL, convert it to an attachment ID
		$logo = attachment_url_to_postid( get_theme_mod( 'logo' ) );
		if ( is_int( $logo ) ) {
			$custom_logo = $logo;
		}
		remove_theme_mod( 'logo' );
	}
	
	set_theme_mod( 'custom_logo', $custom_logo );

}
add_action( 'after_setup_theme', 'prefix_update_check' );