With a seismic overhaul of the $2.6 trillion money-market industry weeks away from kicking in, money managers are bracing for a last-minute exodus of as much as $300 billion from funds in regulators' cross hairs.

Prime funds, which seek higher yields by buying securities like commercial paper, are at the center of the upheaval. Their assets have already plunged by almost $700 billion since the start of 2015, to $789 billion, Investment Company Institute data show. The outflow has rippled across financial markets, shattering demand for banks' and other companies' short-term debt and raising their funding costs.

The transformation of the money-fund industry, where investors turn to park cash, is a result of regulators' efforts to make the financial system safer in the aftermath of the credit crisis. The key date is Oct. 14, when rules take effect mandating that institutional prime and tax-exempt funds end an over-30-year tradition of fixing shares at $1. Funds that hold only government debt will be able to maintain that level. Companies such as Federated Investors Inc. and Fidelity Investments, which have already reduced or altered prime offerings, are preparing in case investors yank more money as the new era approaches.