Track trials used in certain levels to disable those trials when "upgrading" to a new level with a trial.

pmpro_customizations.php

<?php
/*
Plugin Name: PMPro Customizations
Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Customizations for Paid Memberships Pro
Version: .1
Author: Stranger Studios
Author URI: http://www.strangerstudios.com
*/

// set Levels with free trials
global $levels_with_trials;
$levels_with_trials = array(1, 2, 4); //these are the membership level IDs with free trials

// track when a user signs up for a level with a free trial
function my_pmpro_after_change_membership_level($level_id, $user_id) {

    global $levels_with_trials;

    //are they checking out for one of the levels with a trial?
    if(in_array($level_id, $levels_with_trials))
        update_user_meta($user_id, 'pmpro_trial_used', 1);      //track in user meta
}
add_action('pmpro_after_change_membership_level', 'my_pmpro_after_change_membership_level', 10, 2);

// remove trial from level if the user has already used a trial
function my_pmpro_checkout_level($level) {

    global $current_user, $levels_with_trials;

    //only do this for the specified levels
    if(!in_array($level->id, $levels_with_trials))
        return $level;

    //if they already used the trial, disable it for this level
    if(!empty($current_user->pmpro_trial_used))
        $level->trial_limit = 0;

    //return the new level
    return $level;
}
add_filter('pmpro_checkout_level', 'my_pmpro_checkout_level');

//update level cost text if user has already used the trial
function my_pmpro_level_cost_text($cost, $level)
{
    global $current_user, $levels_with_trials, $pmpro_currency_symbol;

    //only do this for the specified levels
    if(!in_array($level->id, $levels_with_trials))
        return $cost;

    //check if the trial has already been used
    if(empty($current_user->pmpro_trial_used))
        return $cost;

    //remove trial from cost
    $cost = "The price for membership is <strong>" . $pmpro_currency_symbol . $level->billing_amount . " per Month.";

    return $cost;
}
add_filter("pmpro_level_cost_text", "my_pmpro_level_cost_text", 10, 2);