Override the default is_downgrade check in the PMPro Proration Add On

pmpro_is_downgrade_custom_filter.php

/**
 * Override the default is_downgrade check in the PMPro Proration Add On
 * Add this code to a custom plugin.
 * A rank of level orders is used to calculate downgrades from upgrades.
 * Be sure to change the $level_order array below as needed.
 * You can also do your own calculation on the $old_level and $new_level objects.
 */
function pmpro_is_downgrade_custom_filter( $is_downgrade, $old_level, $new_level ) {
	// this array contains all of the level ids in order of downgrade -> upgrade
	// change this as needed
	$level_order = array(4, 5, 6, 1, 2, 3);
	
	// figure out where the levels rank
	$old_level_rank = array_search( $old_level->id, $level_order );
	$new_level_rank = array_search( $new_level->id, $level_order );
	
	// make sure we have a rank for both levels, otherwise skip to let the plugin guess
	if( $old_level_rank !== false && $new_level_rank !== false ) {
		if( $old_level_rank > $new_level_rank ) {
			$is_downgrade = true;
		} else {
			$is_downgrade = false;
		}
	}
	
	return $is_downgrade;
}
add_filter( 'pmpro_is_downgrade', 'pmpro_is_downgrade_custom_filter', 10, 3 );