Don't let existing members checkout for a free level. [Paid Memberships Pro]

my_pmpro_registration_checks_stop_free_checkouts.php

/**
 * Don't let existing members checkout for a free level.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_registration_checks_stop_free_checkouts( $okay ) {
	global $pmpro_level;

	// Bail if some other check failed already.
	if( ! $okay ) {
		return $okay;
	}

	$free_level_id = 1;		// Set to the id of your free level.
	if( pmpro_hasMembershipLevel() && (int)$pmpro_level->id === (int)$free_level_id ) {
		pmpro_setMessage( "Your existing level has access to everything the Free level does. If you'd still like to change your level, please <a href='" . esc_url( pmpro_url( 'cancel' ) ) . "'>cancel first</a>.", 'pmpro_error' );

		$okay = false;
	}

	return $okay;
}
add_action( 'pmpro_registration_checks', 'my_pmpro_registration_checks_stop_free_checkouts' );