You can usually change the return date to preserve the length of a trip, this is more of a factor for shorter weekend breaks in Europe. You can also amend a booking for £35 if there is availability. Obviously you probably want to hang on a bit before making changes for something so far off, whilst watching out for loads in case the flight sells out. But you should be able to change the outbound, and the inbound change should be free if it's within a day or two of the original date (but you can't change the route), worst case £35 if longer than that, so long as there is availability for the inbound (can change the route).