I can add to Bill D said about name changes. I play with a guitar player who started in the '40 playing the country music of that day. He occasionally plays with people who claim to play "bluegrass" (in Southwest Missouri/Souteast Kansas). They often comments that what songs he knows and how he plays is not that much different from how they play, only a little slower. Also, many old-time country musicians who sing and play old-time country music in our area will play in C and G; easier for others to follow along and usually only involves three chords. Sometimes people singing more modern country will sing in A, Bb, or E. Fiddle tunes are usually in A or D. I really can't speak for professionals, only relate what we "playin' for fun" people do.