Conversation so far:
What was happening in 1977?
Rick Savage, Tony Kenning, and Pete Willis, all students at Tapton School in Sheffield, South Yorkshire, formed a band called Atomic Mass in 1977.
Did this turn into Def Leppard?
The band adopted a name proposed by Elliott, "Deaf Leopard.
Why did they do this?
in at least partial reference to Led Zeppelin.

Question: What year did the name change happen?
Self-contained rewrite: What year Rick Savage, Tony Kenning, and Pete Willis rename the band Atomic Mass to Def Leppard?