It's the most sweeping change for U.S. money market funds in over three decades, and the biggest operators say it'll have a permanent effect on the way investors allocate their capital.

After years of wrangling with regulators, the $2.7 trillion industry will give up its rock-solid, dollar-for-dollar guarantee for institutional funds that invest mainly in riskier, non-government debt.

The impending change has been a boon for the U.S. government and comes at the expense of banks and other corporate borrowers. Already, investors have shifted more than $1 trillion away from so-called prime funds that buy certificates of deposit and company IOUs and flooded into government-only funds, which invest in Treasury bills and other short-term U.S. debt and are exempt from the change. Assets in those funds, which never exceeded 40% before December, now account for 77% of all money-market assets, according to Investment Company Institute data going back to 2007.