The fact that you have paid extra for flexible flight ticket only entitles you for the right to the change the flight dates, it doesn't mean you'll be able to do so for free, as there's usually a changing fee, and also a difference of cost with the new dates.

As flight tickets are a function of demand and availability (among others), the new dates you are choosing might cost more than your original tickets, hence you'll need to pay the difference so the actual cost would be = changing fee + the new flight date ticket price.

Sometimes the total cost of changing the flight date, even when you've added the extra payment of flexible dates, would cost more than canceling the flight and booking a new one.