We didn't have any charge to amend ours. I don't know whether that was because we only changed the return flight, but we had no charges at all when we did it. We originally booked before the flights were released as well, but I kept watching the flight prices as they were released then changed them in the sale.
You can check by doing online quotes nearer the time to see if you'd benefit from changing, then just phone up if you see flights cheaper.
Yeah, you'd have to book accommodation, but the saving you might make might pay for that, it did with us. You can't really say what you'd save until the flights are released though, but it's definitely worth checking at the time and when they have a sale on. I don't know what flight prices are like for September, but if they were cheaper when you booked, they're probably cheaper in general.