Just saw this posted on DD and should save folks a bunch of $$. You can essentially book a cheap flight around your dates of travel and then switch to a more expensive date of choice for absolutely free, no change fee or difference fee.
Dates from 4/14-6/6 should work on most routes for free changes. Free changes will be allowed within 2 weeks of your eligible flight.
For example I made a test booking from LGA to New Orleans and was able to book an $106 flight and change for free onto a $426 flight.