The CBS network, finally weary of the long domination of early-morning airtime by NBC's "Today" show and ABC's successful upstart, "Good Morning America," yesterday announced that its own "CBS Morning With Charles Kuralt" will expand to 90 minutes next September.

As a key part of the new network strategy, "Captain Kangaroo," which has been on the network since October 1955, will be cut 30 minutes and moved to the low-audience 7 a.m. time slot next fall.

"CBS Morning" thus will start at 7:30 and run until 9 a.m., a time period during which national audiences steadily increase as late sleepers move to their TV dials.

The longest-running network children's series, "Captain Kangaroo" will also be revamped to broaden its current audience of preschool children to include school-age viewers, a move reflecting studies that indicate the children's population overall is no longer growing rapidly as it was a few years back.

The budget for the revised "Kangaroo" will actually be larger than the one currently required for the one-hour program that airs at 8 a.m., according to CBS.

In making the announcement yesterday, CBS Broadcast Group president Gene F. Jankowski also stressed that the "Captain," Bob Keeshan, will become a daily contributor to the network's new 4 p.m. newsmagazine show, which will also debut on the network the week of Sept. 28, at the start of the new CBS season.

In that expanded role, on a late afternoon program aimed at a considerably older audience, Keeshan will address parents regarding their responsibilities, according to Jankowski.

Jankowski said yesterday that the weekly five-hour increase of CBS News programming represented by the restructured "CBS Morning" show and the new half-hour afternoon magazine " probably represents the biggest single weekly addition of news in a given week in our history."

The audience count for the one-hour CBS News program at 7 a.m. over the years has always trailed NBC's "Today" by a large margin. And when the first version of "Good Morning America" debuted on ABC five years ago, and soon carved out a viewership just as large as the NBC program, it was clear to CBS and its affiliates that something would have to be done eventually to challenge the competition, even though the hard news approach of the CBS product induced a certain professional pride at the network.

By expanding "CBS Morning" by 30 minutes and moving it to the 7:30-to-9 a.m. time slot, Cbs will automatically gain in total audience. But whether its current hard news format will make much of a dent in audience share reamins to be seen.

For instance, "CBS Morning" consistently attracts about a 17 percent share of the TV sets-in-use around the country between 7:30 and 8 a.m., the half-hour period monitored in the first morning hour by A.C. Nielsen, compared with 28-to-30 precent audience shares for both "Today" and "Good Morning America."

More importantly, to network sales people, the 7:30 audience for CBS is currently about 3 percent of the total TV homes in the country, compared with about 5 percent for both the ABC and NBC shows in that half hour.

In the 8:30-to-9 a.m. half-hour, also monitored by A.C. Nielsen, those total homes rise to at least 6 percent for the two shows as more viewers turn on their sets, while "Captain Kangaroo" during that half-hour has only 3 percent of the TV homes and a 15 share.

It is that bigger pool of potential audience from 8 to 9 a.m. that CBS News and Network sales obviously hope to dip into, regardless of whether the "CBS Morning" format can increase its share of the sets-in-use, at least in the beginning.