I've done what you say regarding booking beyond the published date a few times when my time off was more restricted and wanted to grab Xmas fares at the best price, but the last time was a few years ago. If you're leaving in the first 10 days or so of December it's not so critical as the price hikes don't normally start until after that. Ask them to note on the booking that one free change is being made available and they should see it on the follow up call.

When you change, you need the same fare bucket to be available but as there's only one for Business Saver that's easy to check for your intended return date by doing a dummy booking.

I wouldn't use a travel agent for this unless it was done face to face with something in writing - too much to go wrong.