I really don't consider asking a customer to start making their mortgage payments payable to the surviving bank's name a "change in the payee" since the surviving institution can certainly accept payments that happen to come in post merger made out to the nonsurviving institution. Besides, customers are being notified of the merger in one form or another if they haven't already heard about it in the news.
This is my opinion on the issue.
If the payee's name is changing from ABC Bank to XYZ Bank then there is a change in the payee, regardless if XYZ Bank continues to accept checks payable to ABC Bank for a period of time after the merger.
Press releases, marketing materials and here-say of the merger do not meet the servicing transfer disclosure requirements.
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The opinions expressed are mine and they are not to be taken as legal advice.