When a user's level is changed to not have a membership, instead give them the free level

cancel_to_free_membership.php

<?php

function my_pmpro_after_change_membership_level( $level_id, $user_id ) {
  // TODO: Replace '1' with level id of free level
  $free_level = 1;
  
    if ( $level_id == 0 && empty( $_REQUEST['delete_account'] ) ) {
        //cancelling, give them level 1 instead
        pmpro_changeMembershipLevel( $free_level, $user_id );
    }
}
add_action("pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level", 10, 2);