You don't mention where you're flying out of. From your earlier posts and the 12,000 mile number, I'm guessing the UK.

Though there are no flex fares ex-LHR that I can see, they are available ex-CDG.

It may be a lot easier though to pay the fare difference to upgrade to a flex fare, if the conditions on changing fares are not very restrictive. Alternatively, try booking something with a return 6-8 months out. That probably won't be a saver. Then you'll still need to change the return date, however.