"Flat Tyre" rules are an American thing.

If you miss your KLM flight, and make no attempt to contact them, it will be cancelled in its entirety. Even if you do attempt to contact them, it is unlikely that they will do anything other than apply the rules of your ticket.

KLM has changed its fare structure quite a lot over even the last few years - but for at least some of the last 2-3 years, even the most basic economy fare was changeable (for a fee, plus fare difference - so it would be quite expensive to change now).

It sounds like at the moment, the cheapest fares are again completely non-flexible.

Don't forget to investigate other airlines for a cheaper last-minute flight.