Python 76 6263 Bytes
This anon-function basically prints it's input as is.
lambda x:`[x[( e~~e,e)[e<len(x)/2]]for e in range(len(x))]`[2::5]
Outputs - z("123456") -> 123456
But by changing the ~
to whitespace , it prints the second part of the string in reverse.
lambda x:`[x[( e~e,e)[e<len(x)/2]]for e in range(len(x))]`[2::5]
^
Outputs - z("123456") -> 123321