From September 18th, employers must start using a new Form I-9 with a revision date of 07/17/17.  As you might guess, the new form was issued on July 17th, by the U.S. Citizenship and Immigration Services. The new form can be downloaded from the USCIS site,

Employers can and, indeed, must continue using existing storage and retention rules for I-9s, which are also on the USCIS site.  I-9 compliance is a priority for the USCIS, and non-compliance is a very serious matter.