Just to be sure...once a flight is in the booking system (72 hrs ahead) you cannot change the ticket price, you can only change the price of flights not yet in the booking system.
Building on what ianmanson said, the prices on the flights that have already been booked will remain the same. However, the flights that will be booked AFTER you changed the price, will now have the updated price.
You can check that on the inventory page in the "data" section, which will show you flights which have been finished or flights that have been booked. Once the flights with the new price have been booked, you will see the flights with the new price booked in the "data" section, and the old flights will still have the old price.
Before changing prices, click all necessary check boxes.
Thank you very much for your answers