<?php
$forms_query = new WP_Query;
$profile_forms = $forms_query->query( array(
	'post_type' => 'um_form',
	'meta_query' => array(
		array(
			'key'   => '_um_mode',
			'value' => 'profile'
		),
	),
	'posts_per_page' => -1,
	'fields' => 'ids'
) );

foreach ( $profile_forms as $form_id ) {
	$profile_forms_fields = get_post_meta( $form_id, '_um_custom_fields', true );

	foreach ( $profile_forms_fields as $key => $field ){

		if( isset( $field['metakey'] ) ){
			$metakey = $field['metakey'];
			if ( $key != $metakey ){
				$profile_forms_fields[$metakey] = $profile_forms_fields[$key];
				unset($profile_forms_fields[$key]);
			}
		}

	}

	update_post_meta( $form_id, '_um_custom_fields', $profile_forms_fields );
}