In principle, you should be able to change the credited FF program (or programme; apologies, I'm a Yank) for any leg you have not yet flown.

In practice, as posted, this requires manual intervention and is therefore not always successful. It seems that different airlines record this information in different parts of their databases, sometimes in more than one part. The part that determines what happens at the end of the day isn't always the part that the agent changes (or can change at all).