Everything to be played by the National Symphony tonight, Wednesday and Thursday, has been changed from the program originally announced by guest conductor Jerzy Semkow and violinist Viktor Tretyakov. Mozart's Symphony No. 36 will open the program, Tretyakov will play the First Violin Concerto by Prokofiev, and the Schumann Fourth Symphony will close the concert. The switches were from Mozart's "Seraglio" Overture, the Shostakovich Concerto and the Schumann Second.

The tragic death of Garnell Stuart Copeland a month ago took one of Washington's most brilliant organists and able church musicians. A concert in his memory, called "Be Joyful!" will be played in the Kennedy Center Concert Hall at 8:30 p.m., Sunday, Feb. 27. Among the organists playing will be Paul Callaway, Reilly Lewis, William Watkins and William Whitehead.

The choir of Epiphany Church, where Copeland was organist-choirmaster, will sing. Free tickets for the concert are available from the National Park Service desk in the Kennedy Center, Ephiphany Church or St. Alban's Church House. The Kennedy Center is a very appropriate place for the concert, for Copeland, who was one of the first organists to play the Filene Memorial Organ in the Concert Hall, often played there during the Thursday noon demonstration concerts.

Metropolitan Opera regional Auditions for this part of the country will take place on Friday and Saturday, Feb. 18-19, in the auditorium of the National Gallery of Art. The finals on Saturday, open to the public, begin at 2:30 p.m., with the doors open at 2:20. Lucine Amara, soprano of the Metropolitan, will be the judge on Saturday.

Four young pianists will play recitals in the National Gallery Sunday on the next four Sundays evenings. They are Robin McCabe, Eugine Rowley, John Bell Young and Gulsin Onay. McCabe's program will include the first Washington performance of Dinu Lipatti's Sonatine; Rowley's the first local hearing of music critic Harriett Johnson's Questions; and Onay's the Washington premiere of Ersin Onay's Four Preludes and two etudes by Saygun.

Erich Leinsdorf, who will guest conduct the National Symphony the week of April 5, has just accepted the post of chief conductor of the Radio Symphony of West Berlin, formerly held by Ferenc Fricsay and Lorin Maazel.

The Washington Children's Opera Theater will give performances in the Chautauqua Tent of the Kennedy Center on Feb. 17 and 18 at 10:30 a.m. and 12:30 p.m., and on Feb. 19 at 10:30 a.m. and 1:30 p.m. For free tickets; contact the Alliance for Arts Education, 254-3250.

Coming competitions: Baltimore Opera Guild, with prizes totaling over $5,000, plus performances: deadline for application, May 2. For details, write Mrs. Alfred C. Ver Valen, 5005 St. Albans Way, Baltimore, Md., 21212. Singers between 20 and 32 are eligible.

American Opera auditions: Deadline March 4. For details write American Opera Auditions, Inc., Carew Tower, Cincinnati, Ohio 45202.

Young Concert Artists, Inc., international auditions: Open to players of piano, violin, viola, cello, string quartets, flute, clarinet, harpsichord, classical guitar and voice. For details write Edna Landau, Young Concert Artists, Inc., 75 East 55th St., New York City 10032.

Reminder: Deadline for the Beethoven Society Pianist Competitions is March 1. For details, write Mrs. Clarence Fisher, 5000 Klingle St. NW, Washington, D.C. 20016.

Auditions For the Oratorio Society's concert of French Choral music, to be given in the Kennedy Center. Call 762-7692.