Hitz

I had the same issue. I spoke to someone in HR about filling out a new I-9 and all she said was 'if you want to' so I'm gonna assume it's not a requirement. Like someone said, the I-9 proves you are eligible to work at the time you are employed and onboarded.

Harpa is right: you don't need to have a new I-9. They can just simply change your status. USCIS won't kill you for a couple month of "non-resident" alien (hence "single"). They know you were in the process. Your tax just going to be adjusted in the end. Congratulations on your green card!

You can update your I-9 if you are concerned about it. You will need to do this anyway prior to the date your EAD would have expired. The I-9 has nothing to do with your tax withholding that would be the W-4. You can withhold anyway you wish. I withhold at the married but single rate and file taxes as married filing jointly. All the tax withholding status does is determine how much money is withheld from your paycheck and goes to taxes. If too much is taken out you should get a refund when you file. if not enough is taken out you have to pay when you file.

You don't have to do a new I-9. But you can.