Google has announced an auto-upgrade for all legacy Forms created in the old version of Google Forms (all forms created prior to February 2013) to the new Google Forms. The new Google Forms supports higher response rates and other advanced features, which we announced in More Improvements with Google Forms.
All responses from the previous Forms will migrate to the new version, though there is potential for some visual differences with old Form themes and stylings. Once the upgrade is complete, a notification will appear when an "editor" opens any upgraded Form alerting the editor that the upgrade has occurred. Google recommends updating to a new theme when alerted of the upgrade to ensure a smooth experience with the Form for users. In the future, Google will also be auto-converting legacy themes to comparable versions in the new system, which will allow theme customization and an improved mobile experience.
Google is also launching add-ons for Google Forms. We've posted several announcements about add-ons and the features that add-ons bring to Google Docs and Spreadsheets ( Add-ons for Docs is Here!, Add-ons Now Have Installable Triggers, What's New with Add-ons), so be sure to check out the growing list of available add-ons in Forms.