Will Koppel change careers? The man who decided at age 9 to become a broadcaster now has reservations. With a contract renewal looming, ABC's `Nightline' anchor notes changes in television that make a newsman's job both more difficult and less satisfying.

``Tell Koppel to announce for the presidency,'' reads the letter from New York Gov. Mario Cuomo, ``and I'll be secretary of state.'' It is addressed to David Burke, executive vice-president at Capital Cities/ABC News, and Ted Koppel has posted it on the door of his ``Nightline'' office here at ABC News headquarters in Washington, across the street from the stately Mayflower Hotel.

The letter is real, but the idea is a joke. Or at least an exaggeration.

Two months ago, Newsweek ran a cover story on Mr. Koppel, in which he indicated that in 1975, then-Secretary of State Henry Kissinger had offered him the post of assistant secretary of state for public affairs, which he turned down. Today, he said, he'd want a different post, but the idea of something within the State Department appeals. In the same article, Marvin Kalb suggested Koppel would make ``an outstanding secretary of state.'' Thus the Cuomo letter.

Now, we are sitting in Koppel's office, a room littered with stacks of mail and books piled all over the desk, tables, and windowsills. Two of the books: ``What's the Good Word,'' by William Safire, and ``Keeping in Touch,'' by Ellen Goodman. There's a huge hourglass on the desk, a Marine Corps Marathon poster on the wall, and a red Chinese banner hanging behind the velvet sofa.

It is 5 p.m., and Koppel and his staff are preparing for the evening ``Nightline'' broadcast at 11:30. He has removed his jacket and hung it behind the door on a hanger marked ``Person to Person,'' which, he explains, is a souvenir from the guest room of Edward R. Murrow. ``By the time I was nine years old, living in England, listening to this rich, deep baritone on BBC, I had already decided that this was what I wanted to be, a foreign correspondent. It was all based on this image of Murrow.''

Koppel's parents had fled Nazi Germany in 1938, and he was born in England in 1940. When he was 13, his family emigrated once more - this time to New York City. He earned an undergraduate degree in speech at Syracuse University and a graduate degree in journalism at Stanford before he was hired by ABC in 1963 - at age 23 America's youngest network correspondent. Now, almost 25 years later, his most recent ABC contract will expire Dec. 3.

He indicates that the attitude of Capital Cities/ABC toward news in general is going to be the major factor in future contract negotiations. ``As difficult as it may be for the public at large to grasp the point,'' he says, ``there comes a time in your life and in your career when the easiest thing to negotiate is money. That's not tough. If the only thing that separated us were money, the contract would be resolved in 24 hours.''

But these days Koppel is concerned about other things. ``I fear we may be reverting to old standards,'' he explains. ``When I first joined ABC almost 25 years ago, it was not uncommon for announcers to be doing news. Someone else would write it and hand it to some man - it was rarely, if ever, a woman then - who had a particularly nice voice, and that person would then read it.

``As the demands upon us become greater in terms of time, the very thing that distinguishes ... my generation of newspeople, Peter Jennings, Dan Rather, and Tom Brokaw - namely that we did our own work, our own reporting, our own writing - is becoming more difficult to accomplish. It becomes more difficult to sustain, because of the pressures of technology and economics.

``Networks have discovered that programs like `60 Minutes,' `20/20,' and `Nightline' are not only economically feasible to be put on,'' he continues, ``they are economically profitable. So there is pressure to generate more product and to use the most highly recognizable anchor people to do those products, in addition to their regular, daily work on air. Well, there simply isn't enough time.''

Koppel, in addition to anchoring ``Nightline'' and ``Viewpoint,'' has recently substituted as anchor on ``World News Tonight'' when Peter Jennings was on vacation, has co-anchored the ``Jennings/Koppel Report,'' and is co-anchoring, with Jennings and David Brinkley, a three-hour ABC documentary on the Constitution, airing tonight (8-11). ``We don't have time to do the research or writing on that - just to read the script carefully and make some suggestions. It worries me that we don't have the time.''

Isn't his own high salary an integral part of the problem?

``Precisely,'' he responds. ``They're paying us a lot of money. We are high-visibility personalities. Therefore they feel they must get as much out of these people as they can. From an economic point of view, that makes good sense. From a journalistic point of view, it does not.

``Twenty-eight years ago, when I was covering Vietnam, I would do a piece and send it back. Perhaps 24 or 36 hours would transpire between the time I had done the piece and the time it went on the air. If I needed to, I could always modify a script during those 36 hours and then feed it on a radio line to change something. Now, we have instantaneous news, where we are covering events live, where people are required to call upon whatever reservoir of experience they have to help edit while something is on the air live. That troubles me less when it is done by people like Peter Jennings, who has a world of experience, than when it is done by some of the younger correspondents and anchors who have not a whole lot in the reservoir.''

So where does Koppel envision he will be in the next few years?

``I don't know. Look, the changes in economics and technology are going to change the impact that the networks have....

``There is only one thing that keeps some of us still working at the network, and that is a commitment on the part of the network to do things in a more professional fashion, to use the extraordinary resources that only exist at a network. That includes overseas bureaus; that includes a lot of old and tried-and-true producers and correspondents, who may not be as pretty as they once were but who sure have institutional memory, who can put things into context; that includes a vast library of hundreds of thousands of tapes that some of these newer organizations simply do not have.

``If all the networks are going to be doing is looking at the bottom line, then they are going to find that, for a while, they will seem to be competing economically very well. But there will be a gradual erosion, and I would not want to be part of it.''

Koppel says he's not interested in moving into politics himself. As for other news people doing so, ``in principle, it scares ... me, because I can see how easily it can be misused if someone who has absolutely no background in the field would go into it simply because a lot of people recognize him on the tube. But it seems to me that there is something to be said for having somebody at a fairly high level of an administration who is capable of assimilating a lot of information from a lot of different sources, of analyzing that information, and then making that information clear.''

Are we in danger of reaching a point where the public confuses personal popularity with political capability?

``Sure. I don't see how, having lived through seven years of the Reagan administration, the question can still be raised. President Reagan would be the first to admit that the reason he is sitting in the White House today is because he's an extraordinarily accomplished communicator. He's not there because his background is particularly brilliant; he's there because he was able to take what he knows, or what others were able to tell him, and communicate it in a way that was particularly compelling to the American public. Is that good or bad? I leave it to others to judge.''

Would President Reagan make a good anchor on the evening news?

``Oh, he'd be a brilliant anchor. Whether he would make a good interviewer is something else again. But he'd be a great reader, because he conveys an enormous sense of sincerity. As George Burns once said, sincerity is the secret of success in all kinds of communications. If you can fake sincerity, you've got it made.''