As a frequent business flyer (AA Platinum) I can say you have to assume your flight will change without notice. In the case of my business flights, they're typically booked only 1 week in advance and therefor are only changed due to weather. Delays can be for any reason. However, I always check business flights the night before and vacation flights a few times prior.
As far as advanced ticketing for vacations, as stated earlier by someone, the further out you book, the more likely fine tuning of the schedules are. Our vacation in Sep-Oct was changed. As we are traveling on weekdays, the demand (or lack of) can change anything from the schedule to the plane type. We went from an MD80 both ways to a 757. However, I did have to work with AA as our original flight home was cancelled and we were put on a much later flight. An earlier flight was reinstated but we were not moved back as we had confirmed seats on the later. Thus, I had to work with the AA rep to get it corrected. In a case like that, you should have no problem changing.
Just remember a few things to get better results (if you're not a frequent traveler). Investigate all you can on-line first as things are easier when you know the complete impact to your plans. Second, be as polite to the airline reps as possible. They get abused quite a bit from people that don't understand the system or that are encountering serious issues traveling elsewhere. They can make things very difficult if their day is going poorly and you jump on them and makes demands. (i.e. Your region may be beautiful and sunny, but severe weather elsewhere means they might be experiencing a flood of calls from stranded passengers.) :wave: