The Core Reporting API has two versions.
Version 2.4 is backward compatible with the legacy Data Export Version 2.3.
If you are building a new application or maintaining an existing one, we highly recommend migrating to version 3.0.
One of the biggest changes in switching to the Core Reporting API is that you now need to register your applications via the Google APIs Console and use a project ID to access the API.
With this change, we are also announcing the deprecation of the Data Export API version 2.3. This API will continue to work for 6 months, after which all v2.3 XML requests will return a v2.4 response. Also, we plan to terminate the Data Export API Account Feed. All configuration data should be retrieved through the Google Analytics Management API.
See our Data Export API changelog for all the details of the change and read our developer documentation for more details about each API.
If you have any questions feel free to reach out in our Data Export API Google group.
Jeetendra Soneja and Nick Mihailovski, Google Analytics API Team