NBC announced yesterday that Tom Snyder, the host of the "Tomorrow" late-night talk show since 1973, will become the anchorman for the network's soon-to-be revamped "Weekend" magazine show.

Under terms of a new contract, Snyder will continue with his "Tomorrow" duties and that show will be moved to New York later this spring.

In addition, he will host three prime-time celebrity interview specials for NBC during the 1979-80 season.

Snyder will replace co-anchors Lloyd Dobyns, who has been with "Weekend" since it debuted in October 1974, and Linda Ellerbee, who joined the program last fall when it moved to prime time from its late Saturday night slot.

NBC News said yesterday that Dobyns and Ellerbee will be reassigned within the news organization.

"Weekend," now sen at 10 p.m. on Sundays, will end its current run on April 22. It is scheduled to return in the same time slot on June 10, with not only a new anchor, but a new name, a new executive producer-and, for the most part, it will be live instead of taped.

NBC president Fred Silverman yesterday promised that "Weekend" will return as a "much more hard-hitting magazine show," but details about any format changes are still to be worked out.

Since the program moved into Sunday prime time last year, "Weekends" ratings performance has been dreadful: in part because of the poor showing by the rest of the Sunday night NBC schedule; and in part because on Sunday nights it invariably invited comparison to CBS' phenomenally successful "60 Minutes."

Under executive producer Reuven Frank, "Weekend" had a saucy, often self-mocking air to it, compared to the relatively sober-sided "60 Minutes."

That tone and the poor ratings combined to create a great deal of grumbling among NBC affiliates in recent months, and the change announced yesterday was not unexpected.

Paul Friedman, currently executive producer of the "Today" show, will replace Frank, who has been offered several other opportunities at NBC, News, including a series of specials about America. No replacement for Friedman at "Today" has been chosen. Friedman and Snyder have teamed up before. In 1974 the two joined NBC-owned WNBC in New York and brought that station's news ratings from thrid to first place in the nation'4s biggest TV market.

Because of that success, and the steady late-night audiences he attracts to "Tomorrow," Snyder has been in the $600,000-a-year salary range. No details of his financial arrangements with NBC under the new contract were revealed yesterday.

Although some of his stuffier peers in TV journalism consider Snyder too "show-biz" for the news side, the fact is that he has been remarkably successful as a news anchor.

In addition to his New York success, he pulled NBC's Los Angeles station, KNBC up from third to first in the local news race while anchoring there in the early 1970s, and had previously kept KYW-TV in Philadelphia in first place in that market.

Recent rumors had Snyder moving to Abc News, and over the years there has been talk that he eventually will be in line for an anchor spot on NBC Nightly News.