The Senate Foreign Relations Committee gave overwhelming final approval to the Panama Canal treaties yesterday after a last-minute flip-flop on the one major amendment the committee had accepted.

The treaties now go to the Senate floor, where debate could begin next week.

The flip-flop was essentially just a technicality, but it symbolized a potential problem for treaty backers when the full Senate begins debate.

That problem is whether the Senate can substantively change the treaties without forcing a second plebiscite in Panama, where the original treaties were approved by a 2-to-1 vote Oct. 23.

This is what happened:

On Friday, the Foreign Relations Committee approved a new article for the so-called neutrality treaty that incorporated an Oct 14 communique issued by President Carter and Gen. Omar Torrijos, the Panamanian leader.

That communique - and thus the new article - spelled out U.S. rights to use military force to preserve the neutrality of the canal after the year 2000, and to move warships quickly through the canal in an emergency.

On Friday night the State Department telexed the new treaty article to Panama City, where the U.S. embassy passed it on to Torrijos. On Sunday Panama's two chief negotiators of the canal treaties met with a delegation of visiting U.S. senators, lead by majority whip Alan Cranston (D-Calif.). and said the new article would force Panama to hold a new plebiscite.

Both Torrijos and the United States want to avoid a new plebiscite, primarily because they fear it might be lost, and also because it could be difficult to avoid Paramanian changes in the original text if the United States insists on changes.

News of the Panamanians' reaction was cabled at once to Washington, and repeated at midnight Sunday night by the Panamanian ambassador in Washington to the State Department.

The Panamanians said they could accept the substance of the proposed amendment, but not in the form of a new article to the neutrality treaty. Instead they asked that it be called an annex, or that its wording be added to existing articles on the same subjects.

Apparently, Torrijos and his aides thought the inclusion of a new article in the main body of the treaty represented such an abovious alteration of the original document that they could not claim it was the same treaty the citizens of Panama approved on Oct. 23.

Yesterday morning, Assistant Secretary of State for Congressional Relations Douglas J. Bennet met informally with the Foreign Relations Committee and laid out the new problem. Bennet asked if the committee could reconsider its Friday vote, and change the form of the amendment by adding its language to Articles IV and VI of the existing treaty.

(It was the State Department's position Friday that the new article was the best way to incorporate the Carter-Torrijos communique into the treaties. State Department official said yesterday this was a decision made in Washington, without specifically asking the Panamanians.)

The Foreign Relations Committee willingly agreed to this change. Sen. Clifford P. Case (R-N. J.) called it a "small flap" that was "really just a face-saving operation." Other senators agreed that it didn't matter how the communique language got into the treaty just so it got in somewhere.

But Sen. Robert P. Griffin (R-Mich.), the lone opponent of the treaties on the committee, disagreed. The contents of the communique "either changes the substance of the treaty or it doesn't." Griffin said, suggesting that he didn't see why one form would require a plebiscite when the other allegedly wouldn't.

Sne. Jacob K. Javits ;R-N.Y.) was troubled by the lgeal issue too, and asked the committee to meet briefly in a closed-door session to discuss the problem with State Department officials.

In that closed meeting Javits asked the Carter administration to say on the record why it felt the revised treaty could be accepted without a new plebiscite, informed sources said.

Speaking in reply, Bennet made three points, these sources said. First, Torrijos read the contents of the Oct. 14 communique (the basis for the new amendment) to the citizens of Panama on television before they voted 2 to 1 to back the treaties, so in effect they approved it.

Second, it would be unseemly and ill advised for the United States to demand another plebiscite in Panama on the narrow issue of America's permanent right to intervene in Panama or use the canal for its warships without waiting in line in an emergency. This could antagonize Panamanians and other Latin American countries, Bennet suggested.

Third, if the United States was responsible in effect, for reopening the negotiations by demanding a substantive change that required a new plebiscite, it would be hard to stop the Panamanians from insisting on the changes, too.

Javits told reporters afterward that he had been convinced that there was adequate legal basis for accepting the newly amended treaties without a plebiscite.

As a practical matter, according to several international law experts, the important issue is whether Panama accepts the final U.S. instrument of ratification, assuming the full Senate approves the treaties, and whether Panama's instrument of ratification includes the wording voted on in the Senate. If both those things happen, the treaties will be binding in international law, these lawyers siad.

Some future leader in Panama might try to disown the treaties on the grounds that the ratification process was tainted by unapproved Senate changes in the treaties, one Senate aide noted. But if the new treaties are reonounced, the old 1903 treaties which give the U.S. control over the entire canal zone would come back into force, presumably.

Senate Majority Leader Robert C. Byrd (D-W. Va.) said yesterday he was "increasingly optimistic" that the amended treaties would pass the Senate by the neccessary two-thirds majority.

In another development, a hearing was held in U.S. District Court here on a suit brought by 60 members of the House who argue that the President cannot dispose of government property (the Panama Canal and associated facilities) by treaty, but must do so with legislation passed by House and Senate.

The Justice Department argued before Judge Barrington D. Parket that the House members had no legal right to bring the suit, and even if they did their action is premature since the treaties have not been approved by the Senate.