Usually, if there is a schedule change by KLM (usually a delay), then you are entitled to either a free refund or a free switch without payment of difference of fare. You can try calling via phone, but you will need to yell at the people to get this to work if you're calling the regular line from North America because that call center is horrible.

From what's written above, if you upgrade to a flex ticket, then you will only get refunded the amount that was spent to upgrade to the flex ticket, not the entire cost.