Congress is taking a second look at changes it made last year in the procedures for income-averaging, which the lawmakers found is costing taxpayers more than congressional experts first had estimated.

The income-averaging procedures, in effect since 1964, were designed to save taxpayers from a windfall tax bite resulting from a sudden boost in income, such as a promotion or new job.

Instead of paying the full tax bite on the new, higher salary, a taxpayer can compare his latest earnings with his average income over the four previous years, and then pay a lower rate on the difference.

Usually, it works out well.

But because of the changes enacted last spring, many taxpayers are finding their savings from income-averaging have been cut back substantially -- sometimes by as much as $300. All persons who income-average will be hurt some.

The bigger tax bite stems from the change Congress approved last spring in the standard deduction, replacing it with the new "zero bracket" concept that reduces a taxpayer's taxable income by $3,200.

When the tax-writing committees agreed on the new definition of taxable income, they had to figure out a way to make the new system comparable to that of previous years, to allow taxpayers to continue income-averaging.

There were two ways to do it -- one that kept the tax-savings the same but required a more complex set of calculations, and a second that would cut the tax-savings somewhat but allow a much simpler procedure to compute.

In the hustle-bustle of pushing the bill through last spring, the lawmakers voted for simplicity over tax savings. The drive for "tax simplification" was strong then, and the difference in tax loads didn't seem great.

The problem was, the cutback in tax savings proved far greater than congressional experts had estimated -- as taxpayers and their accountants discovered when they began working on 1977 taxes this past month.

The bite hits hardest at up-and-coming young professionals -- ironically, like many of the staff experts on Capitol Hill who underestimated the impact of the new procedure last spring.

It still isn't certain whether Congress will decide to alter the new proceducers. If it did, congressional experts say, taxpayers would be stuck with more complex calculations in future years.

But taxpayer protests have been heavy enough so the lawmakers know their drive for tax simplification hasn't been appreciated in quite the way they though it would be. For returns due April 15, however, the bote stands as is.