Generally that won't work, Avios tickets are supposed to be inflexible within 24 hours of travel. Some outstations will allow moves unofficially, and if there is some good reason that increases the chance (e.g. overbooked flights later on, irrop implications from other passengers, flight delayed by more than 2 hours). At LHR overwhelmingly that isn't going to happen. If your flight is still more than 24 hours away you can still move it, assuming availability, and paying the appropriate fee. If you haven't changed the reservation from first booking you should be able to do the latter online.