[DATE, ex. Wednesday, June 11, 1998] 

[NAME, COMPANY AND ADDRESS, ex.
John Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245] 

Dear [NAME, ex. John Smith],

It has been a pleasure to serve you in the past, and we look forward to doing business with you in the future. 

I am writing you regarding a change in our service.  [MENTION RATIONAL BEHIND CHANGE AND HOW IT MAY, IF AT ALL, BENEFIT CUSTOMER, ex. To better serve you, we have altered our office hours.  By closing on Sundays, our sales support staff will now be able to offer you technical service from 9:00 to 7:00 on weekdays.  We are confident that this change will better accommodate you business schedule.]

Indeed, we will continue to serve you to the best of our abilities.  You can always call me at XXX-XXXX if I may be of any service to you. 

Sincerely,

[YOUR NAME, ex. Jill Jones]