operator-convert-case is a Vim plugin to provide some operators to convert word case.
There are some convertion examples.
To lower_snake_case:
lower_snake_case -> lower_snake_case  
UPPER_SNAKE_CASE -> upper_snake_case  
UpperCamelCase   -> upper_camel_case  
lowerCamelCase   -> lower_camel_case  
To UPPER_SNAKE_CASE
lower_snake_case -> LOWER_SNAKE_CASE  
UPPER_SNAKE_CASE -> UPPER_SNAKE_CASE  
UpperCamelCase   -> UPPER_CAMEL_CASE  
lowerCamelCase   -> LOWER_CAMEL_CASE  
To lowerCamelCase
lower_snake_case -> lowerSnakeCase  
UPPER_SNAKE_CASE -> upperSnakeCase  
UpperCamelCase   -> upperCamelCase  
lowerCamelCase   -> lowerCamelCase  
To UpperCamelCase
lower_snake_case -> LowerSnakeCase  
UPPER_SNAKE_CASE -> UpperSnakeCase  
UpperCamelCase   -> UpperCamelCase  
lowerCamelCase   -> LowerCamelCase  
The MIT License (MIT)