The new GOP tax bill, which was passed in December 2017, made some sweeping changes. Most of those went into effect in the 2018 tax year, for which filing season officially starts on January 28.

The individual tax rates changed, so the IRS updated the income tax withholding tables and encouraged filers to review and update their W-4 forms — which determine how much income tax is withheld from your pay — to make sure you were withholding the right amount under the new legislation.

But a 2018 survey by H&R Block suggests that most Americans never did that: As of November, only about 20 percent of respondents said they had updated their W-4 in response to the tax overhaul. That means about 80 percent didn't.

If you're among those who didn't, you could get a smaller refund than you anticipated. You could even owe money to the IRS.

"Taxpayers who were most affected by tax reform should have made updates to their W-4 in early 2018," Eric Bronnenkant, head of tax at Betterment, tells CNBC Make It. That group includes filers who itemize and households with dependents, for example. "If they did not, they may be in a position currently to owe [the IRS] additional amounts, plus possible interest and penalties."

At this point, adjusting your W-4 now won't have an impact on your 2018 withholding, Bronnenkant says: "An individual can only make changes to a 2019 W-4 currently. There is no opportunity to make any changes for 2018 withholding."

That said, you should update it right away if necessary so you're set for the 2019 tax year.

If you haven't reviewed your W-4, or aren't familiar with the form, you're not alone: More than half of Americans cannot say what a W-4 is, according to a report from NerdWallet.