Most airlines will allow you to make changes with no fee if they reschedule your flight to another time.
So, it if your reservation is several months out, there's a very good chance they will change the departure time of your flight (even if it's only by 45 minutes). So, unless absolutley necessary, when I wish to make a change, I'll keep montoring my booking and if they make time change, then I pick up the phone and call to change the date advising the new time doesn't work.
Also, if you ever want to refund a ticket, the same applies so you should always wait till the last minute to cancel with the airline because you never know when they might make a change.
This also applies to reward travel. For example, with AC, I've found that any trip booked more than 6 months out with a total of 4 or more flight segments has nearly a 100% probability of having a schedule change (even if it's only to a single flight) in the months leading up to departure. I've refunded many Aeroplan awards this way and never paid the fee.