LONDON (AP) — On a gray afternoon in the days before Easter, a dozen or so schoolchildren straggled into a side building at Rochester Cathedral and began their transformation. Off went the jackets and backpacks, on came burgundy cassocks and white surplices. Then they trooped into the cathedral, opened their mouths and sang as one. The youthful gaggle had become a choir, giving voice to a tradition of choral music in the Church of England that has survived largely unchanged for almost 500 years.