Sudden changes in the political parties of the two Communist regimes battling for control of Cambodia have raised questions about the future of a key leader in each camp and stirred diplomatic speculation about the meaning of the moves.

Western and Asian diplomats said they were surprised by the announcement Saturday that Pen Sovan had been replaced by Heng Samrin as the head of the Communist Party of the Vietnamese-installed Cambodian government in Phnom Penh, ostensibly for health reasons. Pen Sovan had been considered the regime's strongman.

In a separate announcement today, the Communist Party of the Khmer Rouge regime that the Vietnamese ousted from power nearly three years ago proclaimed its own dissolution as part of a "new strategic policy." The party was headed by Khmer Rouge dictator Pol Pot.

There was no immediate word from either side on whether Pen Sovan and Pol Pot retained their other high-level positions. Pen Sovan, 46, has been prime minister of the Phnom Penh government. The 53-year-old Pol Pot has held two top military positions, including commander in chief of the Khmer Rouge army.

The timing of the announcements left some diplomats wondering whether some sort of signals were being sent to the noncommunist Association of Southeast Asian Nations (ASEAN), whose foreign ministers are scheduled to meet Thursday at the Thai resort of Pattaya. The main topic of the meeting will be ASEAN-backed efforts by the Khmer Rouge and two noncommunist Cambodian resistance groups to form a loose coalition government to more effectively oppose the Vietnamese occupation of Cambodia.

This government would be formed under the banner of Democratic Kampuchea, the name of the ousted Khmer Rouge regime still recognized by the United Nations as Cambodia's legal government.

The announcement of the dissolution of the Khmer Rouge's Communist Party of Kampuchea said the move was based on an agreement in December 1979 to pursue a "new strategic policy principle which would not be communist or socialist." The statement said the party's Central Committee agreed in April 1980 that the party "did not conform with the present situation and the new strategic principle."

Then in a four-day meeting three months ago, the announcement said, the Central Committee "decided to end completely all activities of the party."

In addition to dissolving the party, the Central Committee announcement declared that the government of Democratic Kampuchea "must continue to conduct the people and our Army throughout the country to fight against the Vietnamese enemy until they are eliminated from Kampuchean territory." The statement added that "all Khmer patriotic people" had the right to pursue this goal according to their own ideas and methods. The announcement was dated Dec. 6 and broadcast today on the Khmer Rouge clandestine radio.

Taken together, these points appeared to lay the ideological groundwork for a coalition in which the noncommunist members would not be beholden, in practice or theory, to a Communist Party apparatus by virtue of joining the Democratic Kampuchea government.

However, Western diplomats tended today to regard the change as more cosmetic than real.

They pointed out that, following communist practice of creating a "united front" to rally support against an enemy, the Khmer Rouge in mid-1979 created the "Patriotic and Democratic Front of Grand National Union of Kampuchea" along much the same political lines as the Communist Party.

"It's fair to say the motives of the Khmer Rouge are questionable," a Western diplomat said of the dissolution. "I don't think the noncommunist groups will be overly impressed."

A Khmer Rouge official in Bangkok said the Patriotic and Democratic Front still exists and is "provisionally" headed by Khieu Samphan, the president of the Democratic Kampuchea government.

The official, who did not want to be identified, said he had not received any information on whether party chief and former prime minister Pol Pot was keeping the posts of chairman of the Supreme Committee of National Army of Democratic Kampuchea and commander in chief. Pol Pot, the leader held most responsible for the brutality, mass murder and destruction under Khmer Rouge rule from April 1975 to January 1979, was dropped from the government in December 1979.

The Khmer Rouge official said announcement of the party's decision in September to dissolve itself was delayed three months to give time to communicate the move to "partisans, combatants and members of the party." He had no comment on whether the announcement was connected with the efforts to form a coalition.

The official insisted that his government had "permanently" abandoned communism and socialism. He said the choice was between these ideologies and "the survival of the people."

He added, "if we attach ourselves to communism, the Kampuchean nation will disappear because it will be swallowed by Vietnam."

The official said he was surprised by the replacement of Pen Sovan as secretary general of the communist People's Revolutionary Party of Kampuchea, noting that the Hanoi-trained party chief and premier had been considered a favorite of Vietnam.

A one-line announcement in a Dec. 5 broadcast by Phnom Penh radio said President Heng Samrin was taking over as party chief because Pen Sovan "must take a long rest for reasons of health."

A Western diplomat said there was "no reason to think Pen Sovan is sick" but no clear sign either of why he might have been ousted. Nor was there any consensus on what the move might mean for the Phnom Penh government's relationship with Vietnam.

In any case, Vietnamese Communist Party chief Le Duan yesterday sent a message congratulating Heng Samrin on his appointment as party leader.

"At worst, the Vietnamese are making the best of a bad thing," one diplomat said. "At best, they planned it all along."