Hope y'all don't mind me jumping in. I'm a
travel agent, and here's how schedule changes and seat assignments (usually) work:
All major and most minor airlines re-vamp there schedules at least 2 times a year, and up to 4 times a year (that's up to every three months!). This means that the earlier you book your flights, the greater the chance that there will be a change. If you book 1 year out, it's 100% certain, it will be changed at least 2x. They ALL make changes in August for travel in the Fall, and usually again in late winter for travel spring/summer. Changes to flights can range from 1 minute to out-and-out cancelations. I've seen changes of 3 hours or more.
(The exception to the above is SW, which as far as I know doesn't change it's schedule, due to the fact that they only open flights up to booking about 3 or 4 months prior to travel anyway.)
If you book through a
travel agency, the airline notifies the agent and they are supposed to tell you. Traditional agencies do this all the time, and very well. An agency will also double check to make sure your seats haven't changed, and if they do, at least get you re-assigned. TA's are restricted to seats available on the plane, and may not get your 1st seats back if the airline moved you, but at least you have something.
But online agencies like Travelocity and Expedia can be hit and miss about notifying you. And yes, these two web-sites as well as many others ARE travel agencies, just on line instead of at the strip mall down the street. Also, your spam filter may catch the notification (it's been known to happen) so make sure they are on your "okay" list.
If you book the ticket through the airline, the airline must contact you for changes greater than 10 or 15 minutes. They ARE NOT required by the FAA to notify you of seat changes or equipment changes. If they do, it's a courtesy.
All this means is that if you book on-line or through the carrier direct, you need to check your flights occasionaly. If you have a "regular" TA (not online) then THEY should let you know and if they don't, fire them and get a better agent!
Yup, it sucks, but chalk it up to de-regulation. The airlines can do whatever they want!