Dynamic pricing is throughout the hospitality industry so it is not unusual for one guest to pay high and another to pay low. I would just leave it alone. There is really no way for the guest to find out that you are reducing the price since prices are tied to the dates of the reservation and you cannot change the price on booked and therefore blocked dates. Also, you are not asking those who paid low to pay more, because you cannot. However, should the guest modify the reservation, it might be a time when the price will adjust.