Sorry I don't have exact dates, but when I was a kid I wrote Chance and got sales pamphlet's on all their rides, one was still the Flying Bobs, the other the Music Fest and in the pictures the panels were 70s style painting.
So I figure late 70s to the 80s, I know Strates had his ThunderBolt in 85 and it was stated as new ride, so maybe they changed over about then.
The first Music Fest that I saw was in 86 and I believe it was Johnny's United Shows.
Then Childress picked one up in late 80s (not sure from who), but it had winter theme like a Bobs, but with Music Fest name, not sure why he never just stuck with the Fest theme.
I will try to find those things from Chance and see if I can see any dates.