To keep this information from being ignored, you should encourage them to update their records right away. Make sure the name address and phone number stand out (even if the phone number hasn't changed).
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Right now -- before you forget -- change your address book, Rolodex and computer address file! Because <reason for move>, we <are moving to/have moved to/are opening a new office at>:
Abc Company is still committed to offering <company strong point>. Please stop by our new location. We look forward to seeing you!
encl: <List of enclosed items goes here>