Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
It is my great pleasure to advise you that [name of new sales representative] will now be representing our firm in your area.
[Name of individual] has been handling our accounts in [locale] for some time and is extremely knowledgeable in the field of [specify].
Your new representative is scheduled to visit your office on [date].
Sincerely,
[Name]