Allow membership level expiration date extensions for level changes.

my_pmpro_checkout_level_extend_memberships.php

<?php
/**
 * Extend the membership expiration date for a member with remaining days on their current level
  * when they complete checkout for ANY other level that has an expiration date.
  * Always add remaining days to the enddate.
  * Pulled in from: https://gist.github.com/3678054
 */
function my_pmpro_checkout_level_extend_memberships( $level ) {
	global $pmpro_msg, $pmpro_msgt, $current_user;

	//does this level expire? are they an existing members with an expiration date?
	if ( ! empty( $level ) && ! empty( $level->expiration_number ) && pmpro_hasMembershipLevel() && ! empty( $current_user->membership_level->enddate ) ) {
		//get the current enddate of their membership
		$expiration_date = $current_user->membership_level->enddate;

		//calculate days left
		$todays_date = time();
		$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_extend_memberships' );

This recipe is included in the blog post on "Allow expiring members to extend their membership on renewal or level change." at Paid Memberships Pro here: https://www.paidmembershipspro.com/allow-members-purchase-membership-extension/