I must admit, I have also had problems, even on a flexible ticket, in going on an earlier flight on the day with KLM. I've heard it's improved, but this used to be typical on a full S-fare AMS-LCY ticket bought online:

Meeting finishes 2 hours early. Call KLM, ask to change, they say it is too late, as I need to present myself at the ticket desk at least 2h before departure, and pay an offline fee, and that it 'usually requires at least 4h notice to reticket on request'.

Get to check-in, ask about the earlier flight, they say I have to go to the ticket desk.

Go to the ticket desk, ask about the earlier flight, they say it is fully booked and they won't waitlist anyone on it.

Go to the lounge, ask about the earlier flight, they say I should go to the gate.

Go to the gate, ask about the earlier flight, they say it is full, but will let me know if there are any free seats.

Flight is fully boarded, a few no-shows, but now 'it is too late to change my ticket'.

Security won't let me leave D6 to return to the lounge.

Gah!