Antal Dorati, turning his National Symphony baton over to guest conductors for the next four weeks, will guest conduct the Philadelphia Orchestra Thursday through Saturday and next Tuesday, playing Haydn, Mozart, Kodaly, Ives and Strauss. Ilse von Alpenheim wll be his soloist in the Mozart Concerto No. 9.

On March 30, 31 and April 1, Dorati will guest-conduct the Cleveland Orchestra.

In his absence, the National Symphony this week will be under the direction of Moshe atzmon, with soprano Phyllis Curtin singing Barber's "Knoxville, Summer of 1915." Atzmon also will conduct hindemith's Symphonic Metamorphoses on Themes by Weber and the Beethoven Third Symphony.

Speaking of orchestras, the New Orleans Philharmonic has chosen a successor to music director Werner Torkanowsky who has resigned as of the end of the current season. He will be followed by Leonard Slatkin, who presently holds the title of principal guest conductor of both the Minnesota and St. Louis Symphonies.

And speaking of the Minnesota Orchestra, that body, anticipating its 75th birthday in the fall of 1978, is offering a prize of $3,000 for an orchestral composition, "not to exceed 30 minutes." Judges will include Aaron Copland, Gunther Schuller, Slatkin, Dominick Argento, music director Stanislaw Skrowaczewski, Dennis Russell Davies and Henry Smith. Details from Minnesota Orchestra Composers Competition, Orchestra Hall 1111 Nicollet Mall, Minneapolis, Minn., 55403.

And speaking of competitions, recent winners in the National Symphony's Young Soloists' Contest were Adrienne Sirkin, pianist, Edward Fraedrich, saxophonist, and Jung Ae Kim, soprano, who recently also won the Middle Atlantic Regional Competition of the Metropolitan Opera Auditions. All three will be heard in forthcoming NSO concerts and on "WGMS Live" in recital.

Phyllis Curtin, this week's National Symphony soloist, will be the guest at this week's Kennedy Center Performing Arts Symposium, at noon Wednesday in the AFI Theater. All the symposiums are free.

The Baltimore Symphony Orchestra will open the Eighth Inter-American Music Festival at the Kennedy Center May 2 with music by Villa-Lobos, Fernandez, Siegmeister and Lewis. All festival concerts will be free to the public.

REMEMBER: Massenet's "Werther" tonight, Friday, and Sunday at the Kennedy Center by the Opera Society of Washington.