President Reagan yesterday described the newly unveiled Soviet arms proposal as a "change in their position" but questioned whether Moscow would follow through on an announced cutback in Soviet missiles targeted on Western Europe.

Reagan's comments spearheaded generally skeptical U.S. responses to the Paris speech by Soviet leader Mikhail Gorbachev.

Those familiar with details of the Soviet offer said the proposed 50 percent cut in certain nuclear forces of the two superpowers could erode U.S. security, and that NATO allies could be adversely affected by the Soviet plan. An administration official familiar with the Soviet proposals called them "extremely sophisticated" and "highly complex."

Another official called them "a repackaging of a lot of things with a new spin -- the 50 percent reduction." An official from one of the most hawkish elements of the administration called the Soviet offer "not a basis for meaningful forward movement and dialogue."

Reagan, answering questions from reporters as he toured a soap manufacturing plant in St. Bernard, Ohio, said Britain and France are free to negotiate separately with the Soviet Union over their nuclear forces in Europe, as proposed by Gorbachev.

However, Reagan sharply questioned Gorbachev's offer to withdraw some of the Soviet SS20 intermediate-range missiles aimed at Western Europe. The president suggested the Soviets might simply move the mobile missiles to Asia.

"To simply drive them up into the Ural Mountains or someplace else and then say that they're not a threat to Europe makes no sense," Reagan said. "They can be brought back any time they want to turn on the gas."

On what has been a key point with the Soviets, Reagan said "we're not going to retreat" from research on his Strategic Defense Initiative. Asked about testing of the planned antimissile defense system -- a step many critics believe could mean U.S. abrogation of the Antiballistic Missile Treaty -- Reagan said, "that goes along with research."

Gorbachev yesterday did not call for a ban on research but a prohibition on testing, development and deployment of defensive systems.

Despite his criticisms, Reagan did not reject Gorbachev's proposals and indicated that U.S. negotiators will examine them further in Geneva. At one point he expressed hope that the Soviets "may have gotten religion" on reducing nuclear weapons.

Comment from congressional figures who have been briefed in detail on the Soviet proposals, and from arms experts inside and outside of the administration, centered on the following points:

*Soviet strategic reductions. The Soviet proposal envisions cutbacks to "no more than 6,000 nuclear charges" (warheads or bombs) on each side, according to Sen. Richard G. Lugar (R-Ind.), chairman of the Foreign Relations Committee. After applying a further limit of 60 percent for each category of weaponry -- land- or submarine-based missiles, or bombers -- the Soviets would be left with the possibility of having 3,600 warheads from their heavy land-based missiles.

Lugar said this number would still threaten the roughly 1,000 U.S. land-based Minuteman missiles. With this arithmetic, Lugar said, "you might not make the world safer" and "arguably could leave us less secure" than at present.

However, an analysis of the same basic data performed by outside researchers following a U.S. briefing suggested that the Soviets would face large-scale reductions in warheads and the ability to destroy missiles and other targets in reinforced silos.

*U.S. strategic forces. As made public by Gorbachev in his speech, the Soviet definition of the "strategic forces" to be reduced by 50 percent is "nuclear armaments capable of reaching the territory of the other side."

Because of the geographical disparity, this includes U.S. medium-range missiles, bombers and even tactical aircraft in or around Western Europe and some in Asia (because they can hit Soviet territory). It would not include Soviet SS20 medium-range missiles or most Soviet bombers (because they cannot reach U.S. territory).

Such a definition of Soviet strategic forces was advanced by Moscow when the Strategic Arms Limitation Talks (SALT) began in 1969. It was eventually dropped after insistent U.S. protests that this was inequitable, according to officials.

Sen. Pete Wilson (R-Calif.), who said he is "enormously disappointed" by the Soviet offer, said this definition of forces to be covered leaves "no mutuality" in the U.S. and Soviet force structures.

"We would be required to leave our NATO allies naked" because of cutbacks in U.S. European forces, Wilson said.

A U.S. official familiar with the complex arithmetic said that, under the Soviet proposal, a large part of the U.S. quota of "strategic" nuclear systems would be consumed by medium-range missiles, bombers and carrier-based aircraft to back up U.S. and NATO conventional forces.

The upshot of this allocation, he said, could be to leave the Soviets with an advantage as big as 2 to 1 over U.S. forces in actual strategic systems.

*Weapons in space. Gorbachev's proposal for "a complete ban on both sides of offensive space weapons" in his Paris speech appeared to be similar in language to Soviet Foreign Minister Eduard Shevardnadze's proposal in a White House meeting last Friday for "a cessation of work" on space strike weapons.

Both those Soviet formulations, as well as the formulation reportedly placed on the negotiating table in Geneva, could be interpreted to ban research as well as observable testing aimed at development of the SDI.

It was unclear from various Soviet comments whether, and to what extent, they mean to ban "scientific research" toward SDI.

Both Lugar and Wilson questioned a tradeoff of Soviet offensive weapons for U.S. offensive weapons, with the United States in addition agreeing to restrain its space defense program.

*Soviet SS20 cutbacks. Reagan said that, "as I understand it, the only proposal they've made is one that would not be destroying any of their weapons -- it would simply be moving them."

When a reporter noted that Gorbachev proposed to destroy "fixed installations" of the European missiles being removed, Reagan said he would "leave that to our negotiators in Geneva."

A State Department official said the number of SS20 missiles in the European theater is "irrelevant" because the missiles are mobile and have a range of about 3,000 miles. The United States and its allies have "always insisted on a global limit," the official said.

*Gorbachev's public diplomacy. Sen. Claiborne Pell (R.I.), ranking Democrat on the Foreign Relations Committee, said the Soviet offer "deserves very serious attention" by the administration, but added that "I hate to see such serious matters made into a propaganda gambit. I don't think we're going to get very far if it's done in the public arena."

Despite Gorbachev's announcement of the main lines of his proposals, the administration continued to decline to divulge full details of the Soviet offer in Geneva.