NEW YORK — It may sound arcane, but a planned overhaul of the way companies keep their books on pensions and retiree healthcare plans could come at a very real cost to workers counting on those benefits.

The changes — likely to begin by year's end — come as a growing number of companies freeze pensions and cut retiree health benefits, shifting risks and costs to workers. In recent weeks, IBM Corp. and Verizon Communications Inc. have joined the list of those announcing they will freeze their pension plans. Yesterday, Alcoa announced it will no longer offer pension benefits to most U.S. salaried employees it hires beginning March 1.

But some experts say new rules requiring companies to more accurately calculate and show the cost of their retirement promises could speed up the move by employers away from guaranteed pensions and other benefits.

"Changing accounting rules can cause companies to change their behavior," said David Zion, an accounting analyst with Credit Suisse First Boston.

Rules now in place give companies cover. Many have made expensive retirement promises without putting aside all the money needed to meet them. But they don't have to fully disclose the shortfalls in their earnings statements or on their balance sheets.

Instead, firms can post very positive numbers based on assumptions about investment returns, when the actual returns would hurt their results.

"If you change those rules you take that protection away and our thinking is a company may have to go out and protect themselves," Zion said.

The question is how quickly that will happen and how transparent it will be given the rapid cutbacks in benefits already under way.

By law, companies can cut retiree health benefits at any time, as long as the changes don't discriminate. They can't yank pensions, but can freeze pension plans. Such moves leave workers eligible for benefits already earned, but halt gains they would have been entitled to in later years on the job. Other firms have closed pension plans to newly hired workers.

Many companies freezing pensions say they are bolstering 401(k) plans, making set contributions while leaving workers to manage for their own retirement. Small firms started the trend, but in the past year some large employers followed suit in freezing pensions for at least some of their workers, including Sears Holding Corp. and Hewlett-Packard Co.

Pensions and other retirement benefits have stirred controversy in accounting circles for years. Critics say while companies made expensive promises to workers, accounting rules let them engage in a shell game and mislead investors about the value of stocks, bonds and other assets held by pension plans. While they can fluctuate widely, the rules let companies smooth the numbers, creating distortions in their balance sheets that can make a whopping liability look like a sizable asset.

That led the Financial Accounting Standards Board — which sets U.S. accounting rules — to announce late last year that it planned an overhaul.

By year's end, FASB says it likely will require companies to report the funding status of pension plans and other retirement benefits — showing how much those plans contain compared to what is owed to workers — on their balance sheets.

A second phase of changes would reach much further and take several years. Those changes would require companies to more accurately measure and report their retirement benefits, and include those costs in calculating their profits.

For some companies, the change in their reported financial condition would be stark.

For example, if GM was forced to accurately show its true benefit costs on its balance sheet, the company's book value — the difference between assets and liabilities — would have been cut from $27.7 billion in 2004 to a negative $18.5 billion, according to Credit Suisse estimates.