To successfully write a letter to a customer regarding a new hire, strike a positive tone. Asking your customer to leave behind an account manager with whom he's forged a relationship might cause the customer some anxiety, so you'll need to show that the company -- and the new employee -- are prepared to successfully manage the transition. Write and deliver the letter as soon as you can, to give the customer as much time as possible to deal with the transition.

Describe the New Employee's Support System

One big focus in the letter is showing the customer that the new employee won't be going it alone as he starts out. Assign one of your company's executives to be the primary point of contact for the customers' account during the transition, recommends Steve Moore, director of the Texas-based human resources firm Insperity. In the letter, tell the customer that you consider her account to be a top priority, and that's why you're assigning a senior-level person to act as a mentor to the new employee throughout the employee's transition to the new job.

Introduce the Team Members

Introduce both the executive or mentor you've assigned to the transition as well as the new employee. Provide a few key details about the new employee and why he's qualified to manage the customer's needs. That might include talking about the employee's work experience, number of years in the business, or his educational background. Mention how the employee's qualifications make him a good fit for the customer's account.

Mention the Outgoing Employee

In the letter, always maintain an upbeat tone about the change, suggests Moore. When you introduce the new employee, questions about what happened to the outgoing employee may come up. If the old employee left on a positive note, mention that the past employee is no longer working at the company, and provide brief details about the reason for the employee's departure. If the employee's departure was negative, Moore says you can answer the client's questions truthfully, but remain positive about the situation. Moore says it's also acceptable to simply say the employee left to pursue another opportunity.

Talk Details of the Transition

At the end of the letter, talk about your transition plan for the new employee. Mention what duties the new employee will be responsible for starting out, and the dates by which that new hire will completely take over the account. Tell the customer that you want to ensure the smoothest transition possible and suggest a meeting with the new employee, the transition manager or mentor, and the customer to facilitate that. Follow up the letter with a phone call to discuss more details with the customer.