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

After years of wrangling with regulators, the $2.7 trillionindustry will give up its rock-solid, dollar-for-dollarguarantee for institutional funds that invest mainly inriskier, non-government debt.

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