<!-- language-all: lang-cxx -->

#C++11, 30 bytes

As unnamed lambda (allowed in Python):

    [](decltype('B')*c){*c='C'-2;} //'c'-2 for lowercase
    
    //Usage:
    #include <iostream>
    main(){
      char c;
      [](decltype('B')*c){*c='C'-2;}(&c);
      std::cout << c << std::endl;
    }


### 33 bytes

Function that assigns to its parameter:

    int f(decltype('B')*c){*c='C'-2;}

    //Usage:

    #include <iostream>
    main(){
     char c;
     f(&c);
     std::cout << c << std::endl;
    }


Pretty hard to do it otherwise since no `#include`, `return` or `auto` because of <s>U</s>. Modifying parameters for return values seems ok [Meta][1]


  [1]: http://meta.codegolf.stackexchange.com/questions/2447/default-for-code-golf-input-output-methods/4942#4942