The natural gas legislation that President Carter has transformed into the life-or-death "centerpiece" of his beleagured energy program is a far cry from the gas problem he first proposed last year.

Indeed, this legislation is much closer to what President once denounced as "war profiteering" than it is to the administration's original gas plan.

Instead of maintaining price control of gas as originally proposed, the new legislation provides for the complete deregulation of new gas by 1985.

In addition, the new legislation lacks some provisions Carter proposed to tilt the anticipated price increases so that more would be borne by industry, less by homeowners.

These two changes in the bill would have two results.

First, prices paid by homeowners would rise more than under Carter's original plan, while those paid by industry would rise less.

Second - partly because industry would not face such large price increases - overall U.S. consumption of gas would not be curtailed as Carter first intended.

Instead of conserving gas, in fact, the bill is now presented as a way of increasing production of natural gas to substitute for imported oil.

The original Carter plan would have eliminated the present two-tier market, in which the price of gas is limited when it is sold across state line and unlimited when it is sold within the state where it is produced. When demand is high, this system encourages producers to sell gas intrastate, creating shortages like the crippling ones of 1976-77 in nonproducing states.

The compromise now before Congress would not guarantee that such shortages could not recur, according to Energy Secretary James B. Schlesinger Jr.

To help explain many of these changes, the administration now offers new rationales for its policies and provides new statistics - some of which have been changed repeatedly - to support its argumentation.

Last November, Carter's press secretary, speaking for the president said, "No bill would be better than a bad bill." Today, administration officials say the compromise measure is not a good bill, but the best obtainable, and that passage of some bill - this one - is crucial to the country's image abroad and to the future of the dollar.

When Carter declared "the moral equivalent of war" in April 1977, he unveiled a plan to reduce U.S. oil consumption - and thus oil imports - in two mains ways. The first was to increase prices by a new tax on domestically produced crude oil. The second was a series of devices to encouraged industry to switch from gas and oil to coal.

Today, the crude oil tax is regarded as dead in the Capitol, and the coal conversion measures have been weakened. Now the largest project saving the administration is predicting would come from the new gas legislation. (in April 1977, the administration predicted that its original gas program would not save any imported oil at all.)

For months Carter called the crude oil tax the element of his energy program, but he has not advanced that view in many weeks. Now he says that the compromise gas bill is the centerpiece of the energy program.

Indeed, only once before has Carter embraced a piece of legislation as fervently as he has this compromised and recompromised gas legislation - the other example was the Panama Canal treaties. The White House has engaged the entire Cabinet and every ally it can find - including some oil companies Carter once vilified - to press for passage of the gas bill which is scheduled to come before the Senate Monday.

The strongest argument the administration is now making for the gas bill is that the world is waiting for a sign that America can take strong action to solve its "energy crisis." Without such a sign, the administration warns, the dollar will take a new beating in world exchange markets.

Carter has called the bill "a symbol of our national will."

Carter has also criticized the gas compromise as imperfect, acknowledging its substantial differences with his original plan. Special Ambassador Robert S. Strauss, a leading administration campaigner for the legislation, has repeatedly criticized it. In one meeting with steel executive, Schlesinger began to argue that the bill was actually a good one, but Strauss cut him off, saying it is not a good bill, but it is the only bill available, according to a source present.

As the content of the gas bill has been altered by House and Senate action, then the secret deliberations of a long-deadlocked conference committee, the statistics used to justify or explain the measure's provisions have shifted, too. These changes in the figures have been a major source of the continuing controversy surrounding the legislation.

For example:

Soon after publication of the compromise gas bill at the end of July, the administration circulated briefing papers on Capitol Hill that said the bill would save 1 million barrels of oil a day by 1985. Soon after that claim was circulated, Carter and Schlesinger told businessmen at the White House that the savings would be 1.4 million barrels a day.

Gas industry spokesman say both figures are essentially guesses that cannot be confirmed. Such predictions depend on elaborate assumptions about the growth of the economy, inflation, industrial use of energy and other varriables, none of which can really be predicted, according to many industry officials.

"Any firm prediction is bull," said one gas industry lobbyist.

Schlesinger last year repeatedly told Congress that raising the gas price above $1.75 per thousand cubic feet would be useless, since that price would induce the maximum possible production out of gas producers.

When Carter assailed the oil and gas companies last fall for "potential wartime profiteering, "he said: "If we tripled the price of oil and natural gas there could be no substantial increase in the rate of exploration" for new supplies.

By last month, however, Schlesinger was arguing that the compromise bill - which would put prices well above $1.75 - would result in higher drilling rates "putting more gas in the interstate market."

Schlesinger's reassessment stems from an alteration of the Department of Energy's basic assumptions about future gas production that was made earlier this year after the gas bill became deadlocked in the House-Senate conference.

The Energy Department's quasi-independent Energy Information Administration has altered its projections about gas prices to make the new compromise appear more favorable to consumers.

Last July the EIA said the compromise bill would raise the price of residential gas to $3.31 per thousand cubic feet - $.34 higher than the price would have been under total deregulation of new gas.

This figure alarmed Rep. John D. Dingell (D-Mich.), an architect of the compromise, and he prodded EIA to recalculate its figures, using new assumptions. A subsequent EIA "further evaluation" concluded that the compromise bill would establish a price for residential gas $.31 lower than total deregulation.

Changes in the administration's objectives began to become evident in December of last year, during the Senate filibuster against the Pearson-Bensten total deregulation bill. At the outset of that filibuster. Carter was telephoning senators, urging them to defeat Pearson-Benson in favor of something close to the original administration plans.

When pro-administration forces concluded that they could not defeat Pearson-Bentsen outright, the White House abandoned its previous position and began to look for compromises. The process of compromise led finally to the unusually complex and detailed compromise bill that comes before the Senate Monday.