An exemplary method for handling passenger requests during elevator system modernization includes modernizing elevator cars over time. The modernized elevator cars are capable of servicing destination requests placed outside of an elevator car and include an indication of a desired destination. The exemplary method includes assigning an elevator car to respond to a new pending destination request according to a selected criterion for selecting between a modernized elevator car and an elevator car that has not yet been modernized. The method includes automatically updating the selected criterion responsive to a change in a number of modernized elevator cars. A percentage of the elevator cars that are not yet modernized are reserved and new pending requests are assigned to a modernized elevator car if a percentage of modernized cars plus the reserved percentage is greater than a current percentage of pending requests assigned to modernized elevator cars.