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).

TIP: depending on where in that hexagone you live: just take TGV to LIlle (EUR)-walk to Flandres (10 mins) and use the hourly Belgian trains to Antwerpen/change there to hourly train to NL-Rotterdam-Amsterdam-these are all like TER-UNreserved. Avoid the very expensive Thalys.

Both In BE and NL all trains run ev 30 or 60 mins-like in the Banlieue around Paris and are best compared to a grand-scale metro with out any REServations or other things the SNCF has invneted to discourage travel.