"We will have to reformulate it and provide some qualitative way of providing an assessment of what time horizon we think is most likely," Jeffrey Lacker, president of the Richmond Fed, told reporters in Virginia on Tuesday.
This reformulation could come as soon as a Fed policy-setting meeting on March 18-19.
On the surface, the shift to a more qualitative, succinct message may seem modest.
(Read more: What's the real unemployment rate?)
But the stakes are high: Policymakers are in the midst of phasing out their massive bond-buying program, but worry that the U.S. economic recovery could stall if financial conditions tighten too soon. To ensure that stimulus still flows, they plan to lean ever more heavily on their promise to investors that a rate hike is far in the future.
Lacker pointed to one option available to the Fed: relying more on charts the Fed publishes four times a year showing when each individual policymaker expects rates to finally rise, and how high they will be up to four years into the future.
The so-called summary of economic projections, or SEP, last published in December, has 12 of the Fed's 17 policymakers expecting to begin to tighten policy some time next year—an expectation that aligns with traders in rate-futures markets.
(Read more: This is what Greenspan is really worried about …)
"I'd point out that the SEP provides a rich portrayal of the array of views within the committee, and even if we said nothing the SEP would be pretty informative," Lacker said.
At its March policy meeting, the Fed could simply erase an extensive reference to its rate-rise thresholds—including a nod to 6.5-percent unemployment and 2.5 percent inflation—and restate that easy policy will be needed "for a considerable time after the asset purchase program ends and the economic recovery strengthens."
Yellen, at her first press conference as chair after the meeting, could then direct investors' attention to the SEP, which also shows Fed officials' forecasts for unemployment, inflation, and economic growth over the next few years.
(Read more: Not only weather to blame for 'weird' jobs report)
As it stands, the central bank's policy is to keep rates near zero until "well past the time" joblessness falls to below 6.5 percent "especially" if inflation expectations remain weak.
The trick for Yellen is rewriting this so-called forward guidance without suggesting the Fed is poised to drop its support for the economy, which could spark a spike in borrowing costs that stalls the slow recovery from the Great Recession.
"What you don't want is for markets to suddenly say, 'the economy is doing better,"' said Paul Ashworth, chief North American economist at Capital Economics, a research firm.
Life before and after thresholds
The idea of using inflation and unemployment thresholds to telegraph rate expectations initially came from Chicago Fed President Charles Evans. He spent more than a year urging colleagues it was more effective than so-called calendar-based guidance, which the Fed had adopted in 2011 with a pledge to keep rates low until mid-2013.
(Read more: Skating close to edge, again, on the debt ceiling)
By late 2012, the Fed had twice revised its low-rate pledge, extending it out to mid-2015. It was then that Evans won a key convert.