Conversation so far:
What was happening in 1977 for Def Leppard?
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 Atomic Mass turn into Def Leppard?
Joe Elliott proposed the name Deaf Leopard which was originally a band name he thought up while writing reviews for imaginary rock bands in his English class.

Question: Why did they do this?
Self-contained rewrite: Why did Atomic Mass change their name to Def Leppard?