Five months after President Reagan's "Star Wars" speech, the Defense Department is accelerating and changing the direction of its $500 million programs to develop laser- and particle-beam weapons to shoot down missiles in space, according to informed sources.

Most significantly, the military is switching emphasis from chemical lasers for short-range tactical use in land, sea or air battles to faster, higher-powered, short-wavelength lasers that could attack nuclear warhead missiles in space.

The research program changes, worked out earlier this month with a House-Senate conference committee finishing the fiscal 1984 defense authorization bill, result partly from what a recent House Armed Services Committee report called "technology breakthroughs in short-wavelength lasers that could demonstrate the feasibility of a viable defensive system within five years."

But the sources said they were prompted primarily by Reagan's surprise announcement last March that he wanted a major effort to create defenses against intercontinental ballistic missiles that would "give us the means of rendering these nuclear weapons impotent and obsolete."

The research program changes include:

* Switching research emphasis from long-wavelength chemical lasers which are created by combustion to visible/ultraviolet, short-wavelength lasers created by electrical or nuclear sources.

* Increasing funds for defensive, third-generation, nuclear-generated laser weapons such as the Excalibur program promoted by Dr. Edward Teller. Under this concept, powerful X-ray lasers are created by a nuclear explosion.

* Closing out the Air Force airborne laser laboratory, which used old chemical laser technology to attack and destroy a Sidewinder anti-air missile earlier this year.

* Transferring the Navy's mid-infrared advanced chemical laser to the Defense Advanced Projects Agency [DARPA,] where it will be used to test the vulnerability of U.S. weapons systems.

* Establishing a $25 million laser test range for the Air Force, Army and Navy at White Sands Missile Range, N.M.

* Cutting $12 million from the Army laser programs, while leaving $20 million to test the concept of a "modest-powered battlefield" weapon called Road Runner, according to one Pentagon source.

The administration's particle-beam program for fiscal 1984 remains unchanged, except for an additional $7 million for the Army to work on a neutral-beam technology. The programs designed to develop an electronic beam from a test accelerator are lagging behind the laser research, according to Pentagon officials.

"We have proof of concept with lasers," one official said, "and know they will work" in an anti-ballistic missile system. The questions about lasers, the official added, "were whether they would be lethal and affordable."

With particle beams, however, "we don't know if we can form a beam" that can destroy an incoming missile or warhead, the official said.

More changes are expected after a committee established by the Pentagon and headed by Dr. James Fletcher, former head of the National Aeronautics and Space Administration, reports to President Reagan next month.

The Fletcher group of scientific experts, which was formed shortly after Reagan's March speech, is analyzing where laser- and particle-beam technologies are today and what type of feasible missile defenses could be created from them.

A second Pentagon study group, headed by Dr. Fred Hoffman of the Institute for Defense Analysis, is looking into the potential impact of such futuristic weapon systems on arms control negotiations, relations with allies, and possible responses by the Soviet Union.

Concern has been expressed among the NATO allies in Europe that Reagan's plan could result in protecting the U.S. mainland from nuclear attack while the Europeans would remain exposed to Soviet medium-range nuclear missiles. Questions also have been raised about how such a plan would apply to existing agreements prohibiting attacks from outer space and limiting development of anti-ballistic missile systems.

"Normally," said one congressional critic of the Reagan program, "these studies would have been before, not after, a speech such as the president's."

When he made the speech, according to White House sources, Reagan specifically avoided consulting with scientific experts to prevent the proposal from being leaked in advance.

At that time, however, the emphasis on U.S. laser research was on chemical lasers, primarily for tactical use but also for anti-ballistic missile defense.

The basic elements in the existing space defense program were the "space laser triad"--three programs called Talon Gold, Alpha and Lode.

Talon Gold includes target acquisition, tracking and precision pointing of a laser. Alpha is a highly efficient, infrared chemical laser. Lode is the program to develop complex mirrors and high performance beam control systems for advanced space lasers.