The short answer to that question is NO. You don't have to remove the existing insulation before adding more. However, removing the insulation allows us to perform a much better job of air sealing leakage points in the attic as they become easier to see and seal. If you have signs of rodents in the attic you definitely need to remove the old insulation as it will contain rodent fecal and urine tracings. These can become airborne and create unhealthy situations in the home.