It depends on your airline. Please click here to check the Terms and Conditions for your airline.
Please note, that for extension purposes, JFK, LGA and EWR are all considered NY, and they may be interchangeable on your extension confirmation.
If your airline permits this change, then you can request it on the Extension request form. On the form just include your requested airport change in the COMMENTS section of the form. The cost of the change is the same as the cost of an extension.