Signaling the long-expected shake-up in the top ranks of ABC, Anthony D. Thomopoulos, president of ABC Broadcast Group, submitted his resignation yesterday, effective immediately.

Sources in New York, where the ABC board met yesterday, indicate that other TV executives at ABC may be departing within a few days. Unlike Thomopoulos, however, they aren't expected to leave the third-ranked network under their own power.

Lew Erlicht, the top programmer for the network under Thomopoulos, is reportedly on particularly shaky ground. Last spring, ABC fell to third place in prime-time ratings for the first time in a decade . . .

Yesterday, speculation about his successor included Lee Rich, president of Lorimar Productions (which produces "Our Family Honor" for the network, as well as "Falcon Crest" and "Knots Landing" for CBS), and Brandon Stoddard, currently the ABC executive in charge of TV films and mini-series . . .

Ironically, the turnovers at the top come on the heels of big ratings for both "North and South" and ABC coverage of the World Series, which together are expected to, momentarily at least, put that network in second place, ahead of CBS, for the new fall season . . .

Thomopoulos, who became president of ABC Entertainment in 1978, was part of the team assembled by Fred Pierce in the mid-1970s that between 1976 and 1979 actually wrested first place from CBS in the prime-time ratings . . .

Thomopoulos' resignation yesterday came as Pierce, now American Broadcasting Companies Inc. president and chief operating officer, announced a restructuring of the company that will place him in direct charge of all five operating divisions of the Broadcast Group, effectively eliminating the job Thomopoulos is leaving . . .

Within a few weeks, the takeover of ABC Inc. by Capital Cities Communications Co. will be completedwork. He subsequently worked in sales and TV production for several companies, joining ABC in 1973 as vice president, prime-time programs, New York . . .

In his statement yesterday, Thomopoulos said he made the decision to leave ABC "most reluctantly, but with a clear sense that it is time for me to move on to other challenges in the entertainment industry. I leave ABC with extraordinary respect for and gratitude to Fred Pierce and Leonard Goldenson, two men who have been important influences on my career, and I leave with great personal respect for those people who have worked with me over the past 12 years in helping ABC achieve such enormous success in the broadcasting industry" . . .

Pierce, in a statement, cited his close relationship with Thomopoulos during the latter's 12 years with the company, adding that "he has made significant contributions to ABC and leaves with my deep affection and my personal wishes for continued success in our business" . . .

In his statement, Goldenson said "Tony has been an outstanding executive in many different programming areas and we will miss him. He has been an important part of our successes and I wish him well" . . . Meanwhile

Around the corner in Manhattan at CBS yesterday morning, James Rosenfield, senior executive vice president, CBS/Broadcast Group, announced that he was taking early retirement under terms of the early retirement offer made in September by the company . . .

Yesterday was the last notification day under the special plan, which was offered to about 2,000 of CBS' 30,000 employes . . .

Rosenfield's announcement reportedly came as a surprise to top executives . . .

He first joined the CBS Television Network as a sales executive in 1965 and in 1977 was named president of the TV network . . .

In that job he was part of the successful team put together by CBS Broadcast Group President Gene Jankowski that eventually won back the prime-time ratings crown from the Pierce team at ABC . . .

In 1981, Dartmouth graduate Rosenfield was named executive vice president of the Broadcast Group in charge of the network, CBS Entertainment and CBS Sports divisions . . .

In a restructuring of the Broadcast Group in 1983, he was named senior executive vice president, responsible for finance, operations and development . . .

At the time there was speculation that Rosenfield was unhappy over being moved out of the "action" jobs involving direct broadcasting . . .

Yesterday, Rosenfield denied this. "The last tw ccess in this exciting new phase of his career" . . .

The CBS board of directors meets Wednesday and the CBS affiliates board meets next week in Laguna Beach, Calif. Rosenfield's departure will probably be a topic of conversation at both meetings . . .