You can lock in today's rate as you wish (though you have no way of knowing if prices will fall further, and if they do you have no recourse -- it's like buying a stock on a given day). If you think you're looking at a good rate now and don't mind the schedule possibly changing somewhat between now and departure day, go ahead.

There is no firm deadline for schedule tweaks -- Delta is right now cutting November departures across the Atlantic by a few percentage points owing to weak bookings, which means hundreds of passengers will have their itineraries changed only weeks before they travel.