Migrate old favicon to new favicon option from core

gistfile1.txt

function bookrev_migrate_favicon(){
 		if ( function_exists( 'wp_site_icon' ) ) {
 			if ( get_theme_mod('old-favicon-image') ) {
 				$id = attachment_url_to_postid( get_theme_mod('old-favicon-image') );
 				if ( is_int( $id ) ) {
 					update_option( 'site_icon', $id );
 				}
 				remove_theme_mod( 'old-favicon-image' );
 			}
 		}
 	}
 
 add_action( 'after_setup_theme', 'bookrev_migrate_favicon' );
 
 It can also be applied to custom logo
 
 function bookrev_migrate_logo(){
   if ( get_theme_mod('old-header-logo') ) {
     $logo = attachment_url_to_postid( get_theme_mod( 'old-header-logo' ) );
     if ( is_int( $logo ) ) {
       set_theme_mod( 'custom_logo', $logo );
     }
     remove_theme_mod( 'old-header-logo' );
   }
 }
 add_action( 'after_setup_theme', 'bookrev_migrate_logo' );