Ultimate Member - Send Welcome Email after Role Change.

mail-role-change.php

<?php
add_action( 'um_after_member_role_upgrade', 'my_um_after_member_role_upgrade', 10, 3 );

function my_um_after_member_role_upgrade( $hook_roles, $old_roles, $user_id ){
   $user = get_userdata( $user_id );
   UM()->mail()->send( $user->user_email, 'welcome_email' );
}