Answers

It's really hard to say because it depends on airline and rules can vary from company to company. Some airlines permit you to change your return date without additional charges, but others can charge an extra fee from $25 to $100. Be aware with selecting your new return route: If there will be no available seats on your selected airfare you have to change your flight again, what results in other taxes and fees.
Regarding the return date, you can extend it even to a year; it says that the best date to book your flight is 330 days prior to the flight date. For more detailed information give your flight details, regards and have a nice trip! - (2008-06-22)