Rip-off: change flight vs cancel + new booking

I just noticed I booked a flight wrong and was looking into the difference of changing it vs rebooking it entirely (I‘m within the 24h free-cancellation period).

When changing the wrong flight, BA wants an extra +£150 (minimum).

While cancelling and rebooking it entirely actually saves £50!!

WTF BA…how can they be so inconsistent…