The United States' Western partners in seeking independence for Namibia are reacting extremely cautiously to an apparent change of heart by South Africa in talks with the Reagan administration on the implementation of an independence plan.

Non-U.S. sources have confirmed that the Pretoria government has indicated it would accept in principle United Nations Resolution 435--which embodies the independence plan drawn up in 1978--and a U.N. peacekeeping force in Namibia, also known as Southwest Africa.

This would represent a considerable shift on the part of the South Africans, whose foreign minister, Pik Botha, made clear earlier this year that they no longer backed Resolution 435. But the other governments in the Namibia "contact group"--West Germany, France, Britain and Canada--are reluctant to attach as much importance to the change as Secretary of State Alexander M. Haig Jr.

In effect, it would mean that South Africa has returned to its position of three years ago, after hardening against Resolution 435 because of concerns about border security and reluctance to accept a leading role for the United Nations because of the General Assembly's vote in support for SWAPO (the Southwest Africa People's Organization) as the sole and authentic representative of the Namibian people.

Neither the State Department nor the South African government will confirm the development.

Detailed negotiations are now in progress for a crucial Sept. 24 meeting of the contact group foreign ministers in New York.

In a London-datelined story yesterday, the Rand Daily Mail in South Africa said a new contact group plan calls for withdrawal of U.N. recognition of SWAPO and looks toward independence by December 1982.

At the same time, according to the Daily Mail, the plan calls for SWAPO forces to assemble in U.N.-supervised camps and the withdrawal of South African forces from the territory, the agreement of all parties that laws relating to property and civil liberties be changed only by a two-thirds majority vote of a future Namibian parliament and a definite SWAPO commitment not to allow guerrillas to attack South Africa from Namibian territory.

The Reagan administration has come under strong pressure from the Organization of African Unity, which told Haig on Sept. 1 that African nations were running out of patience with South Africa and wanted assurances that the United States still is committed to the implemention of Resolution 435.

One of the difficulties in the discussions in the contact group is that the United States is more concerned than the Europeans about tying the Namibia problem to the presence of Soviet and Cuban troops in Angola.

Diplomatic sources suggest that the preparations for the Sept. 24 meeting are making progress, but that too much optimism is unwarranted.

[A U.N. General Assembly emergency session on Namibia was to vote last night on a Third World resolution against South African rule over the territory, but the vote was postponed because of text revisions. The measure is expected to be adopted by a large majority.]

[The resolution says the assembly is "firmly resolved" to "immediately enact" sanctions against South Africa. But under the U.N. Charter, the assembly can only propose sanctions while it takes the Security Council to impose them formally. The United States is likely to veto a Security Council vote for sanctions, as it has in the past.]