President Reagan's Strategic Defense Initiative (SDI), which Soviet leader Mikhail Gorbachev hopes to make the central issue of this week's summit in Geneva, cannot be developed fully without abandoning or negotiating a change in the 1972 Antiballistic Missile Treaty, the director of the program has told Congress.

Reagan has said that his research and development program will not violate the provisions of the ABM treaty, but Lt. Gen. James A. Abrahamson told a Senate Armed Services subcommittee on Oct. 30, "There clearly will come a time in the 1990s when we enter the development phase and . . . require much more direct testing of components of a defensive system that we will have to have a modified ABM treaty in some way in order to proceed . . . . "

Already, Abrahamson said, the treaty's provisions limiting testing and development of antimissile systems have "put restrictions on the pace and direction" of the SDI program and forced modifications in "how one would demonstrate the capability through testing" that would persuade a skeptical Congress to appropriate more funds for the system.

Abrahamson, director of the Defense Department's Strategic Defense Initiative Organization, spoke with unusual candor about the limits the ABM treaty will impose in the next five or six years on any continuing effort to establish the practicality of a "Star Wars" defense. In previously unreported testimony, he indicated that the first hard decisions "downstream" would face Reagan's successor, probably in 1991.

The Soviets have sought to raise the issue of how the SDI research program should be altered or controlled to comply with existing or new arms-control agreements, and they are expected to press it again in Geneva.

In an eve-of-summit letter to Reagan leaked to reporters Friday, Defense Secretary Caspar W. Weinberger cautioned that "the Soviets in Geneva doubtless will seek assurances that you will continue to be bound to . . . tight limits on SDI development and testing that would discourage the Congress from making any but token appropriations" for SDI.

Until now, the administration has said it will make no changes in its SDI program but has offered many times to discuss with Moscow what it calls "the transition" from offensive weapons to defensive ones. As part of that transition phase, Reagan has offered to negotiate about giving or selling Star Wars technology to the Soviets.

But the Soviets have said they are not interested in any such transition or in sharing American technology. They insist on banning development of what they call "space-strike weapons," devices based in space capable of attacking targets on Earth or in flight through space. Moscow has said that any reductions in offensive arms will depend on a banning space-strike weapons.

Although the administration has refused to discuss any limitations on SDI research and development, senior officials have said, if indirectly, that it could be possible to agree to new restrictions on Star Wars with the Soviets while still continuing a research program on defensive weapons. That is what Weinberger appeared to be warning Reagan against in his letter.

On the other hand, as Abrahamson testified to the Senate panel, it will be difficult to demonstrate the viability of a new defense until open tests -- which clearly violate the ABM treaty -- are conducted. Without such tests to demonstrate possibilities, Abrahamson added, "it would be a much more complex and difficult decision" for Congress to decide whether to proceed with development of a defense.

Abrahamson also addressed the difficulties of moving from deterrence strategies based on offensive forces, which both superpowers now pursue, to new postures based on a combination of offensive and defensive arms. "The key," he said, "is to carefully draw down your offensive systems as you are building up your defensive systems," a course that would require full cooperation of the superpowers.

At the same hearing, Fred C. Ikle, undersecretary of defense for policy, was asked if it was "politically naive" to expect such cooperation from the Soviets.

"I think in the short term the hardball view may well be that we cannot get such a transition," Ikle replied.