Well even though I asked the question about R&B, I refuse to get into a muddy debate about the precise definitions of that music genres and others. My stance is to each his or her own when it comes to definitions. As for me, I will continue to call "modern R&B" R&B even if that music has experienced some changes. Heck, I've experienced lots of changes, but I'm still me. (That said, I did change my name. Never mind that) :o)

However, I will quote this comment:

"A musical genre does not simply appear, it gradually evolves to a point in time when some event-performance, publication, or recording allows listeners to perceive its unique qualities and apply a label. Wyonnie Harris' 1947 recording of "Good Rocking Tonight" was one of many "rhythm records" made during the late 1940s, however when it was recorded by Elvis Presley in 1954 it seemed like a new and different approach. What made it seem new and different was its context. Without exploring the history of black popular music, country and western music, race relations, technical developments, and the music business one can be led easily to the conclusion that rock and roll was some new and different music which appeared suddenly.

This page begins with the African musical traits brought here beginning in 1619 and attempts to trace their fusion with the European music brought here by the colonists. The story of this musical interaction is also the story of American popular music and includes the plantation songs of Stephen Foster, the ragtime of Scott Joplin, the blues of Bessie Smith, the jazz of Count Basie, and the jump bands of Louis Jordan. The knowledge of the stream of American popular music allows one to understand that rock and roll was a natural result of the combined forces that affected the music.

http://www.history-of-rock.com/indx.html
Rock and Roll-The Golden Decade 1955-1964