A booking that's week old is not covered by the 24hour role, assuming KLM does observe that rule in Europe.
I have made such mistakes too, but haven't had to pay for those mistakes, as I discovered the mistake within minutes after booking and the 24 hour rule always came to rescue.
As luck would have it, minutes after reading this thread, I made a booking with KLM, on KLM,com and picked the wrong filghts on the inbound. I can live with the ones I chose.
I am on the phone with KLM (Actually Delta). The agent is trying to make the changes as we speak.
A week after the mistake is too long. NO one could help in that situation.