score:6
KLM is likely not tell you much in their reply, other than telling you to go online or call, as availability changes from minute to minute and what they show now is likely to be different when you email them back later.
Your best bet is to log into your current booking and use the change the flight options. You will be able to bail out before any change is made if the flights and costs are not what you wanted.
The second route is to call KLM and let their reservations person check into costs and availability. But KLM does charge an administrative fee for over the phone changes (in addition to any change fee and fare difference).
The costs will be dependent on the class you booked originally. Discounted tickets always come with a change fee attached, full fare tickets usually allow changes without fees. There will also likely be a change in the fare you are paying.
To get a real quick idea of fare costs, simply plug in the date and stops you want and see what the system kicks back out. It won't be 100% guaranteed, but will give you a good starting point for costs. If the fares look OK, then get after the change immediately, as those fares may not be available for long.