Some code for PMPro to handle cases where users will be checking out for the same level but might be switching from recurring to non or vice versa.

my_pmpro_profile_start_date.php

/*
	If checking out for a recurring membership for the level the user already has,
	set the profile start date to their old end date or the date of their next payment.
*/
function my_pmpro_profile_start_date($startdate, $order) {		
		
	//does the user already have this level?
	if(is_user_logged_in() && pmpro_hasMembershipLevel($order->membership_id)) {
		global $current_user;
	
		//get stats for the user's level
		$user_level = pmpro_getMembershipLevelForUser($current_user->ID);
				
		if(!empty($user_level->enddate)) {
			$startdate = date('Y-m-d T00:00:00', $user_level->enddate);
		} elseif(!empty($user_level->cycle_number))	{
			//when is their next payment date
			$current_timestamp = current_time('timestamp');
			$next_payment_timestamp = pmpro_next_payment($current_user->ID);
			
			if($next_payment_timestamp > $current_timestamp)
				$startdate = date('Y-m-d T00:00:00', strtotime('+' . $order->BillingFrequency . ' ' . $order->BillingPeriod, $next_payment_timestamp));
		}
	}
		
	return $startdate;
}
add_filter('pmpro_profile_start_date', 'my_pmpro_profile_start_date', 10, 2);

/*
	If checking out for a non-recurring version of the same level while holding
	a recurring version of the level. Add the days until the next payment to the enddate.
*/
function my_pmpro_checkout_level($level)
{		
	global $pmpro_msg, $pmpro_msgt;

	//does this level expire? are they an existing user of this level?
	if(!empty($level) && !empty($level->expiration_number) && pmpro_hasMembershipLevel($level->id))	{
		//get the current enddate of their membership
		global $current_user;
		
		//only care if the current user has a recurring level
		if(!empty($current_user->membership_level->cycle_number)) {

			//when is their next payment date
			$todays_date = current_time('timestamp');
			$expiration_date = pmpro_next_payment($current_user->ID);
			$time_left = $expiration_date - $todays_date;

			//time left?
			if($time_left > 0) {
				//convert to days and add to the expiration date (assumes expiration was 1 year)
				$days_left = floor($time_left/(60*60*24));

				//figure out days based on period
				if($level->expiration_period == "Day")
					$total_days = $days_left + $level->expiration_number;
				elseif($level->expiration_period == "Week")
					$total_days = $days_left + $level->expiration_number * 7;
				elseif($level->expiration_period == "Month")
					$total_days = $days_left + $level->expiration_number * 30;
				elseif($level->expiration_period == "Year")
					$total_days = $days_left + $level->expiration_number * 365;

				//update number and period
				$level->expiration_number = $total_days;
				$level->expiration_period = "Day";
			}
		}
	}

	return $level;
}
add_filter("pmpro_checkout_level", "my_pmpro_checkout_level");