Place a PMPro member in another level and give them a new expiration date when they cancel.

pmpro_cancelled_level.php

/*
	When users cancel (are changed to membership level 0) we give them another "cancelled" level. Can be used to downgrade someone to a free level when they cancel.
*/
function my_pmpro_after_change_membership_level($level_id, $user_id)
{
	if($level_id == 0)
	{
		//cancelling, give them level 1 instead
		$expiry_level_id = 1;
		$level = pmpro_getLevel($expiry_level_id);
		
		//give them an expiry date one year from today.
		$level->enddate = date('Y-m-d',strtotime(date("Y-m-d", mktime()) . " + 365 day"));
		
		pmpro_changeMembershipLevel($level, $user_id);
	}
}
add_action("pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level", 10, 2);