I recommend support the letters case of the word

E.g. in Python you wish to switch between True and False, in javascript its true, and false, in some other cases its TRUE and FALSE.
One could added extra pairs for each situation, but I think the following would be better:

• If all lower case, switch answer to all lower case
• If all upper case, switch answer to all upper case
• If the first letter is uppercase, and the remining letters lower case, switch answer such that the first letter is uppercase only.
• One defines the pairs in lower case.
• If a pair has some weird casing, e.g. stANge and weIRD, then not handle toggling casing