SUBSTANTIAL RISKS ARE attached to the proposals for Zimbabwe-Rhodesia that the British government has now embraced. It would require Zimbabwe-Rhodesia's Prime Minister Abel Muzorewa to accede to a new constitution drafted by the British, and to new elections supervised by the British. Those are large concessions to ask of a shaky government, two months old, in a country engulfed in civil war. Merely by endorsing the plan, Britain has further diminished Mr. Muzorewa's modest authority.

But if the proposal is risky, it also contains much hope. Mr. Muzorewa and his government are in a bad position and, regardless of who's to blame, they probably cannot rescue themselves. Constitutional change is required. Of the 100 seats in parliament, its law gives 28 to the whites who comprise something under 4 percent of the population; more important, whites retain control of the police, the military and the judiciary. To a great many Africans, the Muzorewa government is hardly more than a veil for continued control by the white minority. Mr. Muzorewa is apparently unable to make basic changes under the pressure of the war - but the war will continue until the changes are made.

The hope in the new plan lies in its sponsorship by the Commonwealth. That includes Tanzania, Zambia and Nigeria - two of them crucial to the region's stability through proximity, the third through its money bags. Together with Britain, these countries have it in their power to confer legitimacy on a future government of Zimbabwe-Rhodesia. Without that blessing, no government there seems likely to survive for long.

In the past, the Carter administration has come perilously close to suggesting that a government of Zimbabwe-Rhodesia can be legitimate only through the participation of the guerillas of the Patriotic Front. But because the guerillas have refused to negotiate on any terms but their own, that policy came to a dead end some time ago. Since the Patriotic Front has always found refuge and support in Zambia and Tanzania, it is fair to assume that those two governments will lend themselves to no solution that necessarily cripples or destroys it. But they are exceedingly anxious to end the fighting, which is becoming an incalculable menace to the entire region. With Commonwealth sponsorship of a constitution written by the British, the Patriotic Front would face a hard choice. It would either have to submit at last to elections, or it would have to sacrifice its own claim to a legitimacy that is its principal asset and its sustenance. If there is still a chance to work toward democracy and peace simultaneously in Zimbabwe-Rhodesia, this plan may well now be the only plausible route.