The AMP plugin will eventually allow you to switch a theme that declares itself as Transitional (paired=true) to also switch to Native. In the mean time, this plugin can achieve the same effect. For more, see https://github.com/ampproject/amp-wp/issues/2312

amp-force-native-mode.php

<?php
/**
 * AMP Force Native Mode
 *
 * @package   AMP
 * @license   GPL-2.0+
 * @copyright 2019 Google LLC
 *
 * @wordpress-plugin
 * Plugin Name: AMP Force Native Mode
 * Description: The AMP plugin will eventually <a href="https://github.com/ampproject/amp-wp/issues/2312">allow</a> you to switch a theme that declares itself as Transitional (paired=true) to also switch to Native. In the mean time, this plugin can achieve the same effect.
 * Plugin URI: https://github.com/ampproject/amp-wp/issues/2312
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * Version: 1.0
 * License: GPLv2 or later
 */

add_action(
	'after_setup_theme',
	function() {
		$support = get_theme_support( 'amp' );
		if ( isset( $support[0] ) && is_array( $support[0] ) ) {
			$support = $support[0];
		} else {
			$support = array();
		}
		unset( $support['paired'] );
		add_theme_support( 'amp', $support );
	},
	PHP_INT_MAX
);