bibtex-autokey-name-case-convert-function

bibtex-autokey-name-case-convert-function is a variable defined in `bibtex.el'.
Its value is

downcase

• This variable may be risky if used as a file-local variable.
• This variable is safe as a file local variable if its value
  satisfies the predicate which is a byte-compiled expression.
  

Documentation:
Function called for each name to perform case conversion.
See `bibtex-generate-autokey' for details.

You can customize this variable.