Re Buddy Holly:
The Brunswick/Coral thing doesn't hold up entirely. For example, ""Rave On" was issued on Coral and is hard rock n roll - produced by Bob Theile, I believe.
The main thing that happened with Buddy Holly's records (this according to John Goldrosen's The Buddy Holly Story and Goldrosen and John Beecher's later bio, Remembering Buddy - the best Holly bios I know of) is that he decided to move to NYC and to break ties with Norman Petty - primarily for business reasons. The Crickets decided to stay with Norman Petty, even though Holly would have preferred that all of them remained intact as a group.
Buddy Holly had recorded a couple of records in NYC, both with the Crickets - "Rave On", and without the Crickets - "Early in the Morning", and felt that a successful future in the music business would come there, rather than in Texas and Clovis, N.M.
Buddy Holly was evidently open to many types of music, and even though the last studio tunes he recorded were done with strings, according to his widow, he was planning on recording an album of Ray Charles style material and talked about about asking Ray Charles to help him put it together.