From wikipedia:
Seeger was one of the folksingers most responsible for popularizing the spiritual “We Shall Overcome” (also recorded by Joan Baez and many other singer-activists) that became the acknowledged anthem of the 1960s American Civil Rights Movement, soon after folk singer and activist Guy Carawan introduced it at the founding meeting of the Student Nonviolent Coordinating Committee (SNCC) in 1960. In the PBS “American Masters” episode Pete Seeger: The Power of Song, Seeger states it was he who changed the lyric from the traditional “We will overcome” to the more inspirational “We shall overcome”.