Concise case changing code
Bash shell, 16 characters

tr A-Za-z a-zA-Z


Sample runs:

llama@llama:~$ tr A-Za-z a-zA-Z
ThIs is a TeSt!
tHiS IS A tEsT!
llama@llama:~$ echo 'Programming Puzzles &amp; Code Golf Stack Exchange' | tr A-Za-z a-zA-Z
pROGRAMMING pUZZLES &amp; cODE gOLF sTACK eXCHANGE


It works in a file, too (of course):

llama@llama:~$ echo 'tr A-Za-z a-zA-Z' &gt; swapcase.sh
llama@llama:~$ echo 'This is a test!' | bash swapcase.sh
tHIS IS A TEST!