SOLVED. SCI-FI: classical era composer time-travels into the future. Spoiler ahead.

[Ted]

I read this at the end of 1980s, likely 1986-88. A fan of a famous classical era composer, who lived a dangerous life and died young, time travels to the moment just before composer's death and takes him into the future, where his illness is cured.

SPOILER ALERT:

to the fan's dismay, the composer doesn't continue writing the music in his old style, but discovers modern music, joins a electronic music band, writes hits in the current sound. He also discovers drugs and other present day spoils, and ends up in the same predicament as his past.

[Ayshe]

Gianni by Robert Silverberg maybe?

[Ted]

Yes! Thank you, Ayshe - this was an earworm in my head for a long time ;)