The U.S. Treasury Department changed a rule last October to allow employees to roll over $500 of unspent Flexible Spending Account money, ending years of a use-it-or-lose it policy. But most workers have yet to reap its benefits.

Only 8 percent of U.S. companies adopted the FSA program this year, according to data from Alegeus Technologies, the largest provider of benefit administration services. But that figure could jump to as much as 50 percent in 2015, predicts Alegeus Executive Chairman Bob Natt.

FSAs allow workers to set aside pretax money for healthcare expenses.

Employees will likely find out if their company is taking part in the rollover program when they get their open enrollment benefit information this fall.

Those offered the new option will be able to place up to $2,500, pretax, in their FSAs, and roll over as much as $500 of unspent money at the end of the year. Those who continue in traditional FSA plans will have to use all their funds by year-end, or when a grace period stipulated by their companies ends.

But the program's participation rate is meager. About 33 million Americans contribute to an FSA each year. That number includes only a quarter of the workers eligible for it at large corporations, according to benefit consultant Mercer, a unit of Marsh & McLennan.

That enrollment could be boosted by the new rollover benefit, Alegeus' Natt says, allowing both employees and employers to benefit from not paying tax on those contributions.

Indeed, there's already some evidence of the new rule's pulling power.

PrimePay, a third-party benefit administrator, which heavily promoted the rollover option to clients last year, saw a 30 percent adoption rate. The companies that participated saw a 17 percent increase in participants and contribution dollars.

"I was a little disappointed at first," says Steve Jackson, PrimePay's senior vice president for strategic development and channel sales. "But then as I saw what Alegeus was finding nationwide, it seemed better."

FSAs can still be a hard sell to employees.