When making modifications to the details in a reservation like number of people or rate, you might want to keep the same price and not change it. There is an option that you need to tick called
Keep old price.
However, in some cases the price might still change even if you select the option.
Why Keep old price doesn't always work?
If you are changing the number of people in the reservation, it might be because there are products included which are charged per person. The price of the night will remain the same but the count of products will change and therefore the total amount will also not be the same.
If you however want to keep the total price same as before, you will need to reprice the reservation from the Pricing tab of the reservation.
To learn more about price changed, check below.