By default PMPro will only "extend" a membership level if you checkout for the same level. This code will make it so PMPro extends any expiring level with any new expiring level. E.g. if you change from a annual plan to a monthly plan it will add 30 days to the end of your existing membership.

my_pmpro_checkout_level_extend_memberships.php

<?php
/*
        If checking out for ANY level with an expiration, 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");
?>