Answer: A Cambiata voice - not to be confused with the term as used in counterpoint (i.e. the changing tone) - is a "changing voice." It specifically refers to a boy's voice that is in the process of changing.

Professor Irvin Cooper developed a whole singing pedagogy based on his belief that music needs to reflect the nature of the voices of young people as they grow.

In Cooper's classification, a boy whose voice has not changed at all yet is a "boy soprano," and a boy whose voice is beginning to change is a cambiata, while a boy in the later stages of change is a baritone. A cambiata voice has not yet dropped an octave, but has a tone quality different from that of the boy soprano. Boys with a cambiata voice may also find it more difficult to perform passages with many quick notes.

Cooper thought that girls, whose voices do not drop an octave as is characteristic with boys, should all be in the same vocal group, but divided as soprano 1 and soprano 2 as they find most comfortable.

As so often happens with music, there is a whole institute, now called the Cambiata Institute of America for Early Adolescent Vocal Music, dedicated to the ideas of Prof. Cooper. He seems to have been a very good teacher. (Thanks to the institute's current director, Dr. Alan McClung, for updating our information).