A report to show membership level changes (upgrades or downgrades) from initial level to current level.

pmpro_reports_changes.php

<?php
/*
	Changes Report for Paid Memberships Pro
	Title: pmpro_reports_changes
	Slug: pmpro_reports_changes 
*/

//update this array for your desired reports. the format is: "report name" => array( initial_level_id, current_level_id ),		
global $pmpro_reports_level_changes;
$pmpro_reports_level_changes = array(
	"Members upgrading from Level 1 to 2" => array( 1, 2),
	"Members downgrading from Level 3 to 2" => array( 3, 2),
);

global $pmpro_reports;
$pmpro_reports['changes'] = __('Membership Level Changes', 'pmpro-reports-changes');
	
function pmpro_report_changes_widget() {
?>
<table class="widefat striped">
	<thead>
		<tr>
			<th><?php _e( 'Name', 'pmpro-reports-changes' ); ?></th>
			<th><?php _e( 'Initial&nbsp;Level', 'pmpro-reports-changes' ); ?></th>
			<th><?php _e( 'Current&nbsp;Level', 'pmpro-reports-changes' ); ?></th>
			<th><?php _e( 'Count', 'pmpro-reports-changes' ); ?></th>
		</tr>
	</thead>
	<tbody>
	<?php
		global $wpdb, $pmpro_reports_level_changes;
		foreach( $pmpro_reports_level_changes as $key => $pmpro_report_level_changes ) {
			$current_level = pmpro_getLevel( $pmpro_report_level_changes[1] );
			$initial_level = pmpro_getLevel( $pmpro_report_level_changes[0] );
			$changes_count_query = $wpdb->prepare("
			SELECT COUNT(mu1.id) 
			FROM $wpdb->pmpro_memberships_users mu1 
			LEFT JOIN $wpdb->pmpro_memberships_users mu2 ON mu1.user_id = mu2.user_id AND mu2.membership_id = %d AND mu2.id < mu1.id 
			WHERE mu1.membership_id = %d AND mu1.status = 'active' AND mu2.id IS NOT NULL", $initial_level->id, $current_level->id);
			$changes_count = $wpdb->get_var( $changes_count_query);
			?>
			<tr>
				<th scope="row"><?php echo $key; ?></th>
				<td><?php 
					if( empty( $initial_level ) ) {
						echo '-';
					} else {
						echo $initial_level->name; 
					}
				?></td>
				<td><?php echo $current_level->name; ?></td>
				<td>	<strong><?php echo number_format_i18n( $changes_count ); ?></strong></td>
			</tr>
			<?php
		}
	?>
	</tbody>
</table>
<?php
}

function pmpro_report_changes_page() {
?>
<h1><?php _e( 'Membership Level Changes Report', 'pmpro-reports-changes' ); ?></h1>
<table class="widefat striped">
	<thead>
		<tr>
			<th><?php _e( 'Name', 'pmpro-reports-changes' ); ?></th>
			<th><?php _e( 'Initial&nbsp;Level', 'pmpro-reports-changes' ); ?></th>
			<th><?php _e( 'Current&nbsp;Level', 'pmpro-reports-changes' ); ?></th>
			<th><?php _e( 'Count', 'pmpro-reports-changes' ); ?></th>
		</tr>
	</thead>
	<tbody>
	<?php
		global $wpdb, $pmpro_reports_level_changes;
		foreach( $pmpro_reports_level_changes as $key => $pmpro_report_level_changes ) {
			$current_level = pmpro_getLevel( $pmpro_report_level_changes[1] );
			$initial_level = pmpro_getLevel( $pmpro_report_level_changes[0] );
			$changes_count_query = $wpdb->prepare("
			SELECT COUNT(mu1.id) 
			FROM $wpdb->pmpro_memberships_users mu1 
			LEFT JOIN $wpdb->pmpro_memberships_users mu2 ON mu1.user_id = mu2.user_id AND mu2.membership_id = %d AND mu2.id < mu1.id 
			WHERE mu1.membership_id = %d AND mu1.status = 'active' AND mu2.id IS NOT NULL", $initial_level->id, $current_level->id);
			$changes_count = $wpdb->get_var( $changes_count_query);
			?>
			<tr>
				<th scope="row"><?php echo $key; ?></th>
				<td><?php 
					if( empty( $initial_level ) ) {
						echo '-';
					} else {
						echo $initial_level->name; 
					}
				?></td>
				<td><?php echo $current_level->name; ?></td>
				<td>	<strong><?php echo number_format_i18n( $changes_count ); ?></strong></td>
			</tr>
			<?php
		}
	?>
	</tbody>
</table>
<?php
}

updated version available here, fixes missing "details" button on current version of PMPro:
https://gist.github.com/LMNTL/e60c35c7176463227e370e51d89f6c32

This recipe is included in the blog post on "New Report to View Membership Level Changes (Upgrades or Downgrades)" at Paid Memberships Pro here: https://www.paidmembershipspro.com/new-report-view-membership-level-changes-upgrades-downgrades/