SEVERAL years ago William Schuman declared Aaron Copland to be the dean of American composers and added that the famous musician from Brooklyn had held that distinction with easy grace for a number of years. Schuman said that when he once asked Copland if eh was not "sated with all the honors," Copland replied, "You underestimate my capacity."

Any great musician who reaches the age of 80, which comes to Copland next Friday, has lived through and been a vital part of important changes in musical styles and in public awarenesss of those changes. Copland, however, has lived through changes more wrenchingthan any in the entire history of music. Not even the 14th century, the era called "Ars Nova," with its startling innovations in musical notation and in concepts of rhythm, equalled the upheavals that occurred in music in the years just before and after World War I and again just after World War II.

Copland was 13 when Stravinsky revolutionized musical thought with "Sacre du printemps." He was 23 and studying with Nadia Boulanger in Paris when Arnold Schoenberg formallyenunciated the 12 -- tone system. When Copland was 48, Pierre Schaeffer came along with his theories about musique concrete . And soon thereafter electronic music, which had been through various experimental stages, burst into full flower. Suddenly music, which up to that time had been an evolving art with clearly traceable roots, became rootless. The most basic laws of harmony, rhythm, and melody, which had been crumbling forseveral decades, were dissolved -- by some -- noise, with and without order, was defined, in some circles, as music.

Throughout those decades Aaron Copland was creating the list of works that have won him the right to be called the dean of American composers. They make a fascinating study of a man in the process of maturing, of selecting and rejecting styles and ideas, a man with an open mind toward the new but a firm conviction concerninghis own best path.

Thus, when he was ready to adopt the 12-tone system into his own writing, something he chose to do in his late 50sin his Piano Fantasy, it was with a variant from the strict rules laid down by Schoenberg. Early in his career Copland made a conscious decision that he would write much music that would be accessible to as wide a public as possible. But he continued always to parallel those compositions that are labeled his "Americana" -- "Billy the Kid," "Rodeo," "Appalachian Spring," "The Tender Land" -- with those of a purelyabstract character -- the "third Symphony, the Piano Variations and Sonata, the Piano Quintet and the later Connotations and Inscape.

Copland's music has occupied a central place in this country's artistic life in a variety of ways. His popularballets have been smash hits since their opening nights four decades ago. "Appalachian Spring" is continually played in each of its three forms: the original chamber orchestra version, the fuller score, and the suite from thelatter, which is the way this favorite music will be heard at the composer's 80th birthday celebration in the Kennedy Center on Friday night.

The final work on that program, however, carries the strongest strain of American history of any of Copland's music. In his "Lincoln Portrait" (which Leonard Bernstein will conduct while the composer himself serves as narrator) Copland introduced words by Abrahanm Lincoln. It is a supreme irony that this work -- which Copland has recently narrated on the West Lawn of the U.S. Capitol and which has enjoyed as narrator the services of Eleanor Roosevelt, Adlai Stevenson, Marian Anderons and a host of well-known actors and actresses -- was banned from performance at the Inaugural Concert for Dwight D. Eisenhower and Richard M. Nixon in 1953.

It is a reminder that Copland has lived through not only music upheavals but social and political storms as well."Portrait" was banned in January 1953 wwhen Rep. Fred Busby of Illinois called the National Symphony Orchestra office and said, "You cannotplay a 'Lincoln Portrait' by Aaron Copland because he is a communist sympathizer."

There it was: the naked threat, the unsupported smear. And it worked! The "Portrait" was withdrawn from the inaugural program. And when the congressman was asked later if his action did not mean that "Rodeo" and "Billy the Kid" and all the rest of Copland would have to be banned from programs played by the Army, Navy, Marine and Air Force Bands, where it was often programmed, he answered, "Indeed, that is something we would have to consider."

In 1953, McCarthyism was at its feverish peak, and Copland was only one of a number of American composers similarly and equally baselessly charged. Today that banning seems incredible to a generation for whom Copland has become a symbol of the finest in American music, a generation that fills every hall where he conducts , to whom his music is an irreplaceable part of their musical heritage, containing some of the richest treasure in our musical history. Copland bore the incident graciously 27 years ago. But its memory lent a special splendor to his narration of "Portrait" on the Capitol lawn a year ago. It will carry no lessimpact when he reads it again next Friday, surrounded by the orchestr withwhich he chose to celebrate his anniversary, while enjoying the conducting of the man who has carried his music to a succession of victories for decades, and before an audience that idolizes him. It will be a fitting tribute to aman who has given this country what Aaron Copland has.