Less than a week after the premiere of ABC's "20/20", the network has dropped novice broadcasters Harold Hayes and Robert Hughes as cohosts of the new magazine show. They will be replaced, effective with tonight's telecast, by TV veteran Hugh Downs.

In a statement accompanying the announcement yesterday, executive producer Bob Shanks acknowledged that the first edition of the program "did not meet our expectations." It was widely greeted inside and outside the industry as a bomb. But Shanks also said, "Our faith in the contemporary news magazine remains strong, and the program will continue to evolve."

"20/20" does not appear on ABC's fall schedule as a weekly program but will be monthly, according to ABC, until 1979, when it is to resume a weekly run. Such plans, of course, are subject to change. "60 Minutes," the CBS prime-time magazine program whose success ABC is attempting to emulate, was on the air for three years before it was moved to its present time slot and became a ratings hit.

The fate of TV newcomers Hayes and Hughes was still vague yesterday. "I hope that Harold Hayes and Robert Hughes . . . will remain as either on-air contributors or as editorial members of the program's production staff," Shanks said, but ABC spokemen said that the future participation of Hayes, a former Esquire editor, and Hughes, the Australian-born art editor of Time magazine, was a matter for future negotiations.

Downs was on location at historical sites in Philadelphia yesterday and could not be reached for comment. But Ruth Downs, his wife, said her husband was contacted by ABC the day after "20/20" premiered last week. Downs was then in New York serving as guest host of ABC's "Good Morning, America" show.

Ironically or not, Downs was for nine years the host of "Today" the show that "Good Morning, America" has tried unsuccessfully to topple from ratings domination of the early morning period. Early in his long TV career, Downs cohosted, with Arlene Francis, NBC's pioneering "magazine" show "Home."

Mrs. Downs said from a Philadelphia motel that she and her husband watched the premiere of "20/20" and "We thought it was quite an interesting show, that it has lots of possibilities."

Downs will remain, she said, as host of the daily public television program "Over Easy," which deals with problems of the elderly and is now in production, though not yet fully funded, for a second season beginning in the fall.

After a long day of meetings yesterday, Shanks said from his New York office, "I made mistakes on the first program. I tried to jam too much in." He said a segment devoted to comedian Flip Wilson's weepy recollection of the time he spanked one of his children might have been "very touching" but only "if we have set it up properly. I jammed it on with a 12-second intro."

He also conceded it was "a tactical mistake" to open the first program with Geraldo Rivera's lenghty expose of greyhound racing and its attendant rabbit fatalities, but said he thought it was "a valid piece to do."

In the fallout after the "20/20" premiere, speculation arose that ABC News and Sports President Roone Arledge had behaved true to his reputation and extensively re-edited the finished program at the last minute. "I had a lot of freedom and support from Roone," Shanks said when asked about this rumour. "He did make changes, but I don't find that intolerable or unique. Roone is a hot personality, and so much more is made out of it than if somebody else did the same thing."

Shanks said tonight's program will not be significantly different in structure or concept from the premiere, except for the absence of Hughes and Hayes. Of the team, Shanks said, "It didn't work, and I didn't want them tarnished. They're both decent guys, and television can scissor people up pretty quickly.I hate to see these two very fine men characterized as clowns," he added, referring to printed criticism of the show.

Also in answer to that criticism, Shanks defended the program's mixture of hard news and light feature material and said it was already common practice at newspapers in their "home" and "style" sections. Anytime you try anything new, you're going to get killed for it," Shanks said. "I'm sure people were outraged by the first use of color photography, or the first use of photography in journalism."