Rules maybe rules, however may depend who you get on the day.
I made a booking for a night in London, mid November, as a surprise for 3 work friends as BE were paying the APD equivalent. Turned out that one of my friends had a wedding and couldn't go the dates booked.
Was raging with myself, but phoned the call-centre. Albeit after being on hold for 17 minutes, explained the situation to the lady, and said the week before, or after would suit, same flight times. Changed with no charge, after speaking with her manager, and an incredibly pleasant attitude.... I was very impressed, as my wallet also sighed relief.