code: def get_change_price(self, plan_old, plan_new, period):
          
          if period is None or period < 1:
              return None
  
          plan_old_day_cost = self._calculate_day_cost(plan_old, period)
          plan_new_day_cost = self._calculate_day_cost(plan_new, period)
  
          if plan_new_day_cost <= plan_old_day_cost:
              return self._calculate_final_price(period, None)
          else:
              return self._calculate_final_price(period, plan_new_day_cost - plan_old_day_cost)
docs: Calculates total price of plan change. Returns None if no payment is required.