For what it's worth, I saw Josh do the string change at the Royal Albert Hall in London back around 1959/60. I have always believed this to be a well rehearsed "accidental break" a regular part of the act.
Josh's bass player on this occasion was Canadian Jack Fallon a long time British resident, session bass man and booking agent. He used to saw out a mean Orange Blossom Special on the fiddle at times too.

Hoot