The reason why a site is migrating is because it is moving to a different server. Can you not provide an option to automatically change/add A records and MX records to forward requests to the new server which are active as soon as migration is complete? Otherwise it's a very long process to go through, domain by domain, adding these. Of course we could wait for a day for the nameserver change to update, but during that time the mail is lost as it has gone to the wrong server. An instant update would mean no mail loss.