Move the account record before the relationship. Have the outgoing rep dictate a written record covering who decides, who influences, commitment history, why the account is at its current price, past problems and their resolution, and what would put the relationship at risk. Introduce both reps jointly by phone or in person, with the outgoing rep explaining the change in terms of what the customer gains and vouching for their replacement specifically. Keep the outgoing rep on significant calls for ninety days, then have a manager check in with the customer directly at six months.