I checked with the United States Citizenship and Immigration Services on this issue. If an employee changes their name due to marriage, divorce, etc you are not required to change their I-9 form. If you choose to do so, this needs to be done in Section 3.
Because you shouldn't be changing their name in your payroll system without seeing a social security card with the new name, you may as well get it all done at once. I would never permit I-9s to sit in the active file with the wrong name because of the problems that can create in the future. What the employer's handbook says is:
"Section 3, Updating and Reverification
Employers must complete Section 3 when updating and/or reverifying Form I-9. Employers must reverify employment authorization of their employees on or before the work authorization expiration date recorded in Section 1 (if any). Employers CANNOT specify which document(s) they will accept from an employee.
A. If an employee's name has changed at the time this form is being updated/reverified, complete Block A."
That leaves it open to when you choose to update/reverify. Given that you will see a document for payroll, you may as well use it for I-9 as well.