It's not really "taxed" at the supplemental rate, they just "withhold" taxes at the supplemental rate.
Your employer has the option to withhold at the regular rate based on your W-4, or a flat 25% for Federal (assuming it is less than $1,000,000). You can ask your employer which method they use.
If they take the flat 25%, changing your W-4 won't do anything. If they base it on your W-4, then changing it WILL reduce what they withhold.
It sometimes takes a while for the payroll department to change the W-4, so if you do change it, you should do it as soon as possible.
