One other tool to consider is the ACH Notification of Change (NOC) transaction. As soon as you have identified the new R&T and account numbers, you should consider creating the NOC transactions that go back through the ACH system to notify the originators. Don't rest on this alone, however. Although NACHA rules require the originator to act upon the NOC, many originators either don't understand the process or don't follow up. It's just another tool.
Another tool is to create a system whereby any transactions coming into your shop destined for divested accounts are stripped off and a new ACH tape created, which can be forwarded to the new entity. You should consider trying this for a couple of months in the interests of mutual customer relations. While these recipients will technically be customers of the new entity, you don't really want them to have a sour taste in their mouths over your failure to make the transition as smooth as possible.
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John Burnett
Cape Cod Bank and Trust