Changes would be allowed subject to the fare type, fare difference and the change fee. You can change to any future date which is available which is typically up to 330 days ahead.

Entering the US as a non US citizen you need to show proof of a flight out of US (and that can't be to Canada or Mexico) and that flight would need to within the visa/entry stamp conditions

But as above the T&C's allow for a repricing and you could find yourself paying again. If you cancel