River Movement for the Old Gods

Hi all, I'm still very new to CK2/the GOT mod so please forgive me if I'm asking something stupid here.



I'd like to make a change to my current save, to allow people with the Old Gods religions to travel along rivers, as those of the Mother Rhoyne faith do.



I opened the religions text file with Notepad++, and added "allow_rivermovement = yes" to what I think was the right place, but it doesn't seem to have worked. Will it only work on a new save, or is there something else that I've done wrong?



Thanks in advance for any advice