The fact that the price is high does not necessarily mean that it is priced as a one-way. Since you already flew the first flight I don't think it would be possible to change this roundtrip into a one-way. Unless they would make it two one-ways but that does not make much sense to me.

When you try to make the change on-line via the KLM website does it show you the same prices? Is there anything in your fare rules that would require you to buy up to a more expensive fare? For example, is there a 3-months max stay on your current flight itinerary? In such case if you were to go beyond those 3 months you may need to buy up to a more expensive fare that does allow a longer stay(e.g. 6 months). Just guessing here, however checking the terms of your original ticket may give clues as to why this would happen.