An example of how to use `pmpro_checkout_level` to redirect and interject communication with users during checkout. Sample taken from an attempt to prevent downgrading.

pmpro-redirect-during-checkout.php

<?php // Do not include in Customizations plugin
/**
 * pmpro_checkout_level Use `pmpro_checkout_level` to redirect and interject communication with users during checkout.
 *
 * @param  array $levels The array created in admin
 *
 * @return array         Output of the array
 */
function pmpro_redirect_during_checkout( $level ) {
	 // Check to see if the current user is downgrading (Gold to Silver or Gold to Free)
	if ( pmpro_hasMembershipLevel( 3 ) && ( 1 === intval( $level->id ) || 2 === intval( $level->id ) ) ) {
		 // Re-direct them to page explaining why they cannot downgrade
		 wp_redirect( home_url( '/gold-downgrade-message' ) );
		 exit;
	} // Check to see if the current user is downgrading (Silver to Free)
	else if ( pmpro_hasMembershipLevel( 2 ) && 1 === intval( $level->id ) ) {
		 // Re-direct them to page explaining why they cannot downgrade
		 wp_redirect( home_url( '/silver-downgrade-message' ) );
		 exit;
	}

	 return $level;
}
add_filter( 'pmpro_checkout_level', 'pmpro_redirect_during_checkout' );

The === in this recipe should actually be == as $level->id is a string.

Hey David, thanks for catching that, and you may recall that I have an aversion to not using strict comparisons, so will you accept the above updated with intval( $level->id )?

Absolutely! I just wanted to leave a comment here with a possible fix in case others stumble across this. Thanks Paul!