President Carter's decision to field a fast reaction force that could fly to such destant places as the Persian Gulf has prompted the Pentagon to change airlift priorities.

Defense Secretary Harold Brown, administration officials said yesterday, has told the Air Force to abandon its medium-range transport and put the money into longer-range plane.

This will mean a new cargo giant to succeed the Lockheed C5A, the only plane big enough to carry the Army's heaviest equipment long distances. The new plane has been designated the CX. "The Air Force didn't protest," said one source in describing the guidance Brown gave at a Pentagon budget meeting last Wednesday. The CX is more likely to be a modernized version of the C5A or a militaryized wide-bodied transport already in production than a new design, Pentagon officials said. But this decision is not firm.

The preference for modifying an existing plane stems from Carter's desire to get more long distance airlift capability in the shortest time.The president underscored his interest in a fast deployment force in his recent television address on Soviet troops in Cuba.

The plane the administration is shoving aside is the AMST (advanced, medium-range, short take-off and landing transport). Boeing and McDonnell-Douglas built AMST prototypes under a program that cost the government about $200 million.

The AMST would have delivered troops and cargo from one battle zone to another within a country rather than flying from one continent to another. AMST would have replaced the Lockheed C130 cargo fleet.

The loss of overseas bases, combined with the administration's worries about conflict in the Persian Gulf cutting off the flow of oil to the West, has given a new sense of urgency to closing the gap in long-distance airlift.

The Air Force already is spending billions to strengthen the wings of the C5A and to enlarge the Lockheed C141 so it can carry more of he Army's heavy weaponry to overseas trouble spots.

Brown's guidance on the CX represents and additional step toward improving long-distance delivery of American troops and weapons.