Upgrading Your Google Maps JavaScript Application To v3 - Google Maps API — Google Developers
Quite a lot has changed between v2 and v3 of the Google Maps JavaScript API. As you start working with the new API, you will quickly find that this is not simply an incremental upgrade. The good news is that we've added a lot of great new features and improved the overall usability of the API from the developers standpoint. The bad news is that upgrading to v3 will require rewriting some of your code. If you're planning to upgrade from the Google Maps JavaScript API v2 to the Google Maps JavaScript API v3, this guide will help you through the process, and call out some of the most common changes for users of the v2 API.