You're not officially married until September, so don't change your forms to say that until then. I'm pretty sure that's not allowed, and you'd technically be lying on a government tax form. Don't borrow trouble. ;)
Otherwise, it's easy to change the withholdings at any time - just talk to your HR department. I've got access to do it myself through my company's payroll management software, but even if our companies require you both to fill out a form, it takes like 5 minutes to print it out, fill it in and drop it with your HR person.