I agree with previous poster. If you book now you will probably find a cheap flight but when you call to make the change the difference in Fare + $150 change fee will likely be more or about the same than you would pay for a walk up fare probably (M,B or H fare bucket) especially if you're flexible on time of flight and airport you're arriving into.

the agent you spoke with definitely gave incorrect information. They will not allow you to change your flight without a change fee though some people have been able to have the change fee waived if you're upgrading in fare or class (from Economy to First, for example).

Welcome to FT and Congratulations!!