You can change your journeys without any problem. You don't even have to add a journey in advance (or activate a travel day in advance); you only have to do that before you board a train.
If you planned to take a train with mandatory reservation, however, then that is a limiting factor. You would then need to change the reservation to another train, which is only possible if seats for pass holders are still available on that train. If changing a reservation is possible, you must often do such a change before the departure time on the reservation, there can be costs involved and the number of changes you can do is often also limited (if at all possible).