1. For which train?
2. Where did you buy it?

Varies per railway and type of REServ-if its for just a seat, accept it as money lost-but sometimes its possible to exchange for other date at counter of THAT railway in THAT country only-and before booked trip has started.

Generally speaking, it is not possible to change the date on a reservation once the ticket has been issued. The only exception to this is Thalys and Italian trains. To bring about this change, please get in touch with our customer service team to re-book your tickets. Remember to include your current reservation number and specify your new preferred date and time of travel. You can sometimes request this change locally at the station too. For all other trains, you will have to cancel the reservation in your account and make a new one. Please note that not all reservations are refundable. 

Eurostar reservations can also be changed, for a fee of €15, on the Eurostar website.

Is that doable up to the day of departure or only up to 7 days before?

I had a feeling it was a lot more within 7 days (possibly 40 euro), but I may be confusing myself.

According to Belgian railways up to the departure time:

https://public-pf.b-europe.com/en/Producten/Details?id=3b9ef0c4-e944-467c-a86c-22d90d6f29a6

So at least when booking through Belgian railways that should be possible.