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AA (not Amex) permits free date changes on award tickets, as long as you do not change the date to one that is less than 21 days from when you originally ticketed the award. So if you find award seats and make the change from the 16th to the 15th or the 17th, the change should be free.
JetBlue often offers low fares between JFK and BUF, and allows one free checked bag. BUF is much closer to Niagara Falls than Toronto is.
Both Amtrak and Megabus provide service from NYC to BUF and YYZ. (Amtrak also serves Niagara Falls.) Check their sites for pricing. Discounts on some Amtrak routes may be available. Check the FT Amtrak forum for details. (There's also Amtrak service from Buffalo to Chicago on the Lake Shore Limited.)
Southwest Airlines also serves BUF, and should offer either nonstop or one-stop service between BUF and MDW.