Lane Venardos, for the past two years executive producer of "The CBS Evening News With Dan Rather," has been replaced in that position by his former deputy, Tom Bettag, CBS News announced yesterday in New York. Venardos moves on to the special-events unit at CBS, where he will be executive producer and deputy director.

Competitors at other networks were quick to attribute the change to lowered ratings for "The CBS Evening News." In recent weeks, "NBC Nightly News With Tom Brokaw" has narrowed the ratings gap between it and the front-running Rather show. The distance between Brokaw and the third-place "ABC World News Tonight" with Peter Jennings has widened.

According to NBC sources, ratings for the "Nightly News" for the period from January to the present are up 8 percent over the same period last year, as are ABC's, while the Rather ratings are down 2 percent. But CBS News spokeswoman Ann Morfogen says that in season-to-date ratings, measured from last September, "CBS Evening News" is down only 1 percent, while NBC is up 8 percent and ABC up 5 percent.

"The ratings don't have anything to do with this," Morfogen said in reference to the change in producers. "Ratings were not the issue." Another CBS source said the combined audience for all three network newscasts is up and so NBC's and ABC's gains do not translate directly into CBS losses.

Dan Rather, reached before last night's broadcast, indicated no dissatisfaction with Venardos. "Lane did a superb job here," Rather said. "He is one of the premier producers in the business. He can come back and do this job whenever he wants."

In an internal memo distributed to employes from CBS News President Van Gordon Sauter and Executive Vice President Howard Stringer, Venardos, 41, was praised effusively for leading the "Evening News" through "two of the most difficult years" in CBS News history.

"He has weathered more storms than Odysseus or Magellan, and has done so cheerfully and courageously," the memo said. Stringer himself is a former executive producer of the "Evening News." His tenure was two years, before that of Venardos, who served four months longer than Stringer did.

Bettag, the memo said, "will help maintain the dominance of 'The CBS Evening News With Dan Rather.' "

"When I took this job, back on January 23, 1984, Van and I agreed this was a 2 1/2-year job," Venardos said yesterday. "I leave this broadcast, coincidentally, never having been in second place. And that's kind of a neat thing to have jotted down on your permanent file somewhere."

Venardos praised his successor, Bettag, also 41, as "a wonderful person and my best friend." Bettag has been senior broadcast producer of the "Evening News" since June 1984. That job will now be filled by Andrew Heyward, who moves up from senior producer.

A holdover from the Walter Cronkite era of the "Evening News," Venardos was Washington producer of the program from 1979 to 1981 and was associate producer for the five years preceding that. Bettag joined the program's staff in 1977, after stints as producer on the "Morning News" and a year in Japan as a Fulbright Scholar.

In a prepared statement, Sauter said: "Today more than ever, CBS News is faced with new challenges in the technology we use and in the stories we cover. These moves will strengthen our ability to report on an increasingly complex world, where journalism and technology meet more rapidly and decisively than ever before."

Shortly after 3 p.m. yesterday, when the announcement was made, Venardos, Bettag, Sauter and Rather met in Stringer's office for a champagne toast. Stringer said later of Venardos: "He's a very good guy. He'll be around."