Place a PMPro member in another level with a 30 day expiration date when they cancel/expire unless they are cancelling from that level.

pmpro_after_change_membership_level_default_level.php

function my_pmpro_after_change_membership_level_default_level($level_id, $user_id)
{
	//set this to the id of the level you want to give members when they cancel
	$cancel_level_id = 1;

	//if we see this global set, then another gist is planning to give the user their level back
	global $pmpro_next_payment_timestamp;
	if(!empty($pmpro_next_payment_timestamp))
		return;

	//are they cancelling?
	if($level_id == 0)
	{
		//check if they are cancelling from level $cancel_level_id
		global $wpdb;
		$last_level_id = $wpdb->get_var("SELECT membership_id FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . $user_id . "' ORDER BY id DESC");
		if($last_level_id == $cancel_level_id)
			return;	//let them cancel

		//otherwise give them level $cancel_level_id instead
		$level = pmpro_getLevel($cancel_level_id);
		$level = json_decode(json_encode($level), true);
		$level['membership_id'] = $cancel_level_id;
		$level['user_id'] = $user_id;
    	$level['startdate'] = date('Y-m-d');
		$level['code_id'] = '';
		
    	//give them an expiry date 30 days from today. change the following three lines to change how many days/months/years away the expiry date is
		$level['enddate'] = date('Y-m-d',strtotime(date("Y-m-d", time()) . " +30day"));
    	$level['expiration_number'] = 30;
		$level['expiration_period'] = 'days';
    
		pmpro_changeMembershipLevel($level, $user_id);
	}
}
add_action("pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level_default_level", 10, 2);