One of the most significant changes is that expired documents are no longer acceptable. The previous I-9 allowed employers to accept expired U.S. Passports, as well as expired identification documents from List B. Note that documents without expiration dates (e.g., social security cards, birth certificates, etc.) are assumed to be unexpired.

Employers do not need to replace previously-completed I-9s with the new version. Any completed I-9 which was valid at the time will remain valid, and must be kept for the required retention period. However, older versions of the I-9 can no longer be used. For example, when an employee must be reverified because employment authorization has expired, employers must use the revised Form I-9 with its new List of Acceptable Documents. An employer may not reverify the employee by completing Section 3 - Updating and Reverification on a previous version of Form I-9.

Please go here to download if in doubt - http://www.uscis.gov/files/form/i-9.pdf

A Spanish verison is also available however it can only be used as reference unless you are in Puerto Rico.