Tell them what the change is and the reasons behind it. If it is good news, encourage them to take advantage of it. If it's bad news or a cut-back, thank them for their understanding.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We want to inform you of an important change in our business. Beginning <date change will take effect>, <description of change>.
<Thank you and encouragement.>
encl: <List of enclosed items goes here>