A change fee is a service fee charged only when necessary - which is not in all cases - it's not part of the taxes and fees associated with the travel component, which are the only taxes/fees that BA will cover for Reward Flight Saver redemptions.

As has been noted twice already in this thread - if you wish to avoid change fees, you can become Gold. Perhaps a more realistic way, though, is to hold off from making a booking until you know exactly when you wish to fly.