This is a very common thing that happens. Airlines change their schedules constantly based upon a variety of things like maintenance, airport construction, etc... (I work for AA, and my job is affected on a daily basis by these changes.) When you book a reservation that much in advance, there are bound to be changes that come up between now and then. Most of the time, the schedule is pretty much set at the 45 day mark... but like I said, things happen.
I know that they will make an effort to contact you in some way (phone, e-mail) to notify you of changes in the schedule. The problem is that you may come across a change before they've had the chance to call you to tell you about it.