HOUSTON (AP) _ Saying ''tradition changes,'' a judge approved a settlement under which Texas A&M University must encourage women to join its band and elite cadet corps, an arrangement a lawyer vowed to appeal.

Regents of the school reacted by issuing a written statement complaining that Texas Attorney General Jim Mattox settled the case without their approval and charging that Mattox ''is not a fighter.''

U.S. District Judge Ross Sterling approved the settlement Thursday.

Melanie Zentgraf filed the lawsuit in 1979, when she was a senior at Texas A&M. Now an Air Force captain, she was not accepted for membership in the Ross Volunteers, a group of elite cadets who serve as the honor guard for the state governor.

''It's over, but it's just begun,'' Ms. Zentgraf said Thursday.

Ms. Zentgraf was shunned by many Texas A&M students and faculty members after she filed her suit. At May 1980 graduation exercises, former President Jarvis Miller shook hands with every student receiving a diploma except her.

She said she never learned why her application for membership in the Ross Volunteers was turned down, but assumed she had been barred because she had been openly critical of the male-only policies.

As far as she knows, she said, no woman has ever tried to join the band ''because they knew they would be publicly harassed if they tried. You lost friends by even talking about it. It will be very difficult for the first woman member.''

''I find it fair, just and reasonable,'' Sterling said of the decree. ''I understand the feelings of the alumni and students on this. There is a great deal of tradition, but tradition changes.''

Aggie regents were irate.

''The fighting Texas Aggie Band is worth preserving as a very distinctive and unique organization unlike any other volunteer organization in the country,'' the regents said in a statement.

John Tyler, a Houston lawyer and 1975 A&M graduate, vowed he would appeal Sterling's decision to the 5th U.S. Circuit Court of Appeals in New Orleans.

Mattox, who decided last year to settle the lawsuit, said Sterling's ruling will ''bring the university's band in line with long-standing practices at other Texas universities.''

About 60 women are currently in the 2,100-member corps. Damages are being negotiated.

In the suit, Ms. Zentgraf asked that the corps be forbidden from blocking the membership of women in such organizations as the Aggie Band; the Ross Volunteers; the Fish Drill Team, a freshman organization; and the Color Guard.