MB Relationships: change field type

register.php

<?php
add_action( 'mb_relationships_init', function () {
	MB_Relationships_API::register( [
		'id'   => 'users_to_posts',
		'from' => [
			'object_type' => 'user',
			'field' => [
				'field_type' => 'checkbox_list',
			],
		],
		'to'   => [
			'object_type' => 'post',
			'field' => [
				'field_type' => 'radio',
			],
		],
	] );
} );