Senate Majority Leader Robert C. Byrd (D-W.Va) yesterday warned that attempts to amend the strategic arms limitation treaty (SALT II) run the risk of sending the treaty back to the bargaining table, where the Soviets most likely would start proposing changes of their own.

The Senate begins the ratification process today with the opening of Foreign Relations Committee hearings on the treaty. Some senators, Minority Leader Howars H.Baker Jr (R-Tenn.) among them, made their support for SALT II conditional on amendments to the treaty text.

The administration holds the position that the treaty cannot be amended and that it would be impossible to reopen negotiations with the Soviets. Soviet leaders have made similar statements, most recently when Foreign Minister Andrei Gromyko called reconvening the talks an "impossibility."

Yesterday, however, Byrd -- who just returned from meetings in the Soviet Union with Gromyko and President Leonid I. Brezhnev -- said the talks could be reopened, but to do so would throw the treaty open to amendments from both sides on issues settled after seven years of negotiations.

"If we'er talking about major changes in the treaty text ... then that opens up the treaty for renegotiation, in which case the Soviets would expect to open up some of their problem areas and would probably demand some concessions where they have already made some concessions," Byrd said no "Face the Nation" (Cbs, Wdvm).

Byrd said that if the talks were reopened, the Soviets would say, "Okay you'er opening up the treaty on this point, we want to open it up on this [other point]; you want a concession on this, wer want a concession on this."

Byrd said, however, that if the treaty were voted on as it stands, "there probably would not be a two-thirds vote there" and it would not be approved.

Byrd suggested the possibility of a compromise between amending the treaty to please its critics and opening the entire pact to renegotiation. That is, the Soviets may be willing to accept conditions of ratification -- "reservations or understandings" -- without demanding renegotiation of the treaty.

Reservations to ratification are conditions the Senate coud impose so that senators' sentiments are understood while the text of the treaty remains unchanged.

Byrd suggested that such reservations might include a condition that the "protocol" period of the treaty not be extended past 1981 without the Senate's approval and a condition that the Soviets not build more than 30 "Backfire" bombers in any year, as Brezhnev has promised.

Byrd went to Moscow last week to explain to the Soviet leadership the Senate's role in the ratification process and the differences between Senate reservations, understandings and amendments to the treaty.

Byrd said yesterday that, after his trip, "I do feel they have a better understanding of amendments to the resolution of ratification that would relieve some of the concerns of senators . . . without altering the treaty text."

Byrd said he believes the Soviets "would be very concerned about any amendments that would bring about the renegotiation of the treaty."