Short answer: you do nothing. You keep using the current form until the government issues a new form (or tell us when to stop using the current form). I know this feels wrong, but it really is the right answer.
Long answer: you still do nothing! USCIS is working on a new form, but may not be done with it by August 31. Even if they are, it is expected that the form itself will not have any changes, but rather the instructions will be slightly amended/clarified. Sometime after (or around) August 31, USCIS will update its website stating which versions are acceptable to use going forward.
If you’re someone who prints off a bunch of I-9s to use throughout the year (which I wholeheartedly do NOT recommend), be sure to discontinue use of the old forms as of the effective date put forth by USCIS. My recommendation is to download a new form each time you need to use it. I know it seems like having several forms ready to go is a time-saving measure, but just remember that you can and will be fined during an audit for using the wrong version of the form. The USCIS website always has the most current version and instructions. If you use an electronic vendor, they should also have the new version updated in your system by the effective date of the new version. Double check!
What about existing I-9s? Do they need to be updated or re-done?
Absolutely not! As long as the version used was the proper version at the time, the I-9 lives on. You never have to re-do an I-9 simply because the version expires (can you imagine?! There are 10+ versions of the form released since November 6, 1986).
Above info current as of August 19, 2019.