The American Ballet Theatre has released ballerina Cynthia Gregory from most of her performance obligations through the end of next week so that she can learn the lead roles in two new ballets. One is scheduled to premiere next week in New York and the other is set for early next month in Miami.

The ABT action averted a showdown between Gregory and the directors of the New World Ballet, a company that has been founded to perform at the New World Festival of the Arts in Miami in June. Howard Dando, executive director of the company, had asserted that Gregory had failed to appear for rehearsals of Norbert Vesak's "Tchaikovsky Dances" and was about to be discharged from the engagement.

Dando and choreographer Vesak both confirmed they had sought the services of superstar Natalia Makarova to replace Gregory, but that Makarova's knee problems prevented her from accepting the offer. They then enlisted the aid of ABT Executive Director Herman E. Krawitz, who arranged to free Gregory from most of her performances.

Gregory's partner in "Tchaikovsky Dances" is to be ABT's Fernando Bujones, who, Vesak said, has been rehearsing the ballet in Manhattan for two weeks without Gregory.

Gregory, who missed 10 weeks of ABT's national tour earlier this year because of a pulled muscle, was also about to be replaced in the lead role in another premiere, Peter Anastos' "Clair de Lune," which ABT has scheduled for next Monday. Anastos originally was contracted to create the ballet for Gregory, but ABT ballerina Magali Messac was taught the role when Greogy failed to appear at rehearsals.

The "angel" of the Gregory rehearsal crunch has been ABT's Krawitz, who yesterday said: "We've rearranged our schedules to suit the realities of this girl's recently overcome physical ailments. There are differences of opinion about how fast she's learning certain roles . . . but she's working as hard as she can.

"I told them all to get together under the reality of today--not what was," Krawitz said of the new works' choreographers. "I told them to just calm down."

Gregory's reduced performance load adds to the problems of ABT, now into the fifth week of its eight-week Lincoln Center engagement, with more than half of its star dancers indisposed, injured or released from most of their performances.

Mikhail Baryshnikov is not performing because of recent knee surgery; Martine van Hamel is out with a broken foot; Natalia Makarova has canceled many of her performances because of recurrent disability caused by her chronic knee injury and Gelsey Kirkland is not now on ABT's roster.