"These are vested benefits," she told BenefitsPro.com. "It's deferred compensation … not a gift from the employer."

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Other legal experts speculated on the possibility of an appeal on constitutional grounds, with a claim that the retiree's property rights were violated.

Under this scenario, pension benefits are seen as "deferred wages." That would make any cut in benefits no different than an employer going into an employee's checking account and taking out the money that the employee earned from the employer.

Contract law theory could also be part of any legal challenge. In such an argument, the plaintiffs would assert that an employee has complied with his or her terms of the contract. They worked and then retired. But the employer now is reneging on their part of the contract by failing to pay the benefits.

Legal experts say it would be hard to block the implementation of the amendment by trying to get a court to issue a judgment ahead of any litigation. Instead, a more likely approach would be to wait until someone loses benefits because of the amendment and then files a lawsuit. At that point, there could also be an attempt at making the case into a class action.

Also, if new government regulations are written in response to the amendment, opponents could mount a legal challenge to the rules, just as is the case with IRS regulations conceived in response to the Patient Protection and Affordable Care Act.

Paul Secunda, the chair for 2015 of the Department of Labor's Employee Retirement Income Security Act Advisory Council, said any likely legal challenges will be tough to mount.

For starters, he noted the amendment includes a provision whereby retirees cannot sue through the anti-cutback rule found in the Employee Retirement Income Security Act.

There are, however, three other potential legal strategies that plaintiffs could attempt, though perhaps with not much more luck.

One relates to "non-delegation" – whereby the government cannot give its power to the private sector to cut benefits. Historically, however, such uses of non-delegation doctrine have been unsuccessful, Secunda said.

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A second approach relates to the taking of property. In this area of the law, the concept is that when the government takes property, it is supposed to provide "just compensation," he explained. However, as long as there was due process, there are no constitutional violations.

And the third area relates to "substantive due process," which requires the government to act in a non-arbitrary and non-discriminatory manner and have a good reason for taking the property. This, too, has minimal chances of success, Secunda said.

Still, "I wouldn't be surprised if there was litigation," he said. "There are a lot of upset people."

Ronald Dean, a California-based attorney who specializes in ERISA law and has been co-chair of the American Bar Association's Labor Section's Employee Benefits Committee, noted that any appeal of the amendment would be very different from recent legal challenges in response to cuts in state or local public retiree benefits.

The Kline-Miller amendment is limited to multiemployer pension benefits, and relates solely to the private sector, he said, noting there are no state constitutional issues involved.

Chicago labor unions this week sued to block the city's plan to cut a $20 billion pension deficit on the heels of a successful challenge to cuts to state pension benefits.

The lawsuit, filed by municipal employee and teachers' unions in state court in Chicago, relies on the same Illinois constitutional provision successfully cited by opponents of a plan to fix the state's $111 billion pension deficit, the biggest gap in the U.S.

Judge John W. Belz in Springfield struck down the state law last month, finding it violated a state constitutional prohibition against reducing public worker retirement benefits. Illinois has appealed.

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