Generally speaking, airlines frown upon this because it can be used to circumvent their pricing policies. Here is what the KLM website has to say about it:
If you do not take all flights or legs of this journey, or in the specified order, extra costs may be charged.
If you just show up in Amsterdam, they might charge you something or even sell you another ticket (at whatever price is available at this point) but you have basically no guarantees. They will probably have canceled your return ticket as well.
Also, if you have an (expensive) fully flexible fare, KLM probably allows you to change the date or add a stopover so there is no point in taking chances, you can just change your booking now. On the other hand, if you have a cheap fare, changes are impossible or expensive by design and airlines are unlikely to be willing to accommodate you as expensive tickets would be less attractive if cheap fares offered the same flexibility.
PS: Flying to Amsterdam one or two days before an intercontinental fligt is called a “stopover” and it can be done with KLM at no extra cost or for a fee depending on the date/fare (but you have to book it that way from the get go, not book a flight and take the train instead).