Just saw this when booking a reward flight online - sounds like they haven't got the tech quite there yet!
From 00:00 British Summer Time (GMT +1) on 28 April 2015 the following conditions will apply;
British Airways, Iberia, their franchises and subsidiaries
You can change from a peak date to another peak date and from an off-peak date to another off-peak date using Manage My Booking.
When you change from a peak to an off-peak date using Manage My Booking, you will be refunded the difference in Avios within 10 days. For an immediate refund, please make the change via your local Service Centre.
You can change a flight from an off-peak date to a peak date by calling your local Service Centre. You will need to pay any additional Avios for a peak date, plus any service fees.