Email notification on changing the Level on Pro-sites

When the customer changed the plan from level 3 to level 1, how can I do that send the mail content that tells them they changed the plan from lv.3 to lv.1

The payment successful message will send a mail in next payment circle, that is fine. But I want to do that when a customer changes his plan, we can tell him right away through send the mail.

How do I do?