/**
 * Fires when the system has to be migrated.
 *
 * @param string $version_from_db 
 * @param string $target_version 
 */
function my_webmention_migrate_callback( string $version_from_db, string $target_version ) {
    // Your code here.
}
add_action( 'webmention_migrate', 'my_webmention_migrate_callback', 10, 2 );

\do_action( 'webmention_migrate', $version_from_db, WEBMENTION_VERSION )