Why did the Bolsheviks rename their party the Communist Party?
Actually you have the question backwards. The Communist Party renamed their party the Bolsheviks party to win the war and then changed the name back to the original
The communist party existed in Russia before the Russian civil war.
The Czar resigned under pressure because of the disastrous results of military in world war II and scandals surrounding the Czarist. The democracy that was founded when Russia withdrew from world war I was challenged by rivalries and discord.
Russia dissolved into civil war. The white Russians that wanted to reestablish the monarchy, several democratic fractions and the communists. The communists shrewdly renamed themselves the Bolsheviks which means the majority party.
Under the Czar the common people were denied education and were open to propaganda. The Communist party was one of the smallest parties at the beginning of the Civil War but were the best organized and committed. Calling themselves the majority party (Bolsheviks) was brilliant bringing many of the peasants who wanted to be with the wining side to support the Communist.
After the Communist (Bolsheviks) won the Civil War they quit calling themselves the Bolsheviks and went back to calling themselves the Communists.