In a certain office productivity suite, pressing Shift-F3 will rotate among the different capitalizations of the current word. This functions implements the same behavior. Note that Emacs already has default bindings for CaseConversion? – downcase-word, capitalize-word and upcase-word. However, this function tries to be “intelligent” to act on the current word regardless of where the point is, and it also tries to implement the “rotate” idea.

This is all a neat idea for a command, but has never been proposed elsewhere. Probably because the “do what I want” commands are adequate.

Also here is a simpler version, which should work in most cases (barely tested):

    (defun cycle-word-capitalization ()
      "Change the capitalization of the current word.
    If the word under point is in lower case, capitalize it.  If it
    is in capitalized form, change it to upper case.  If it is in
    upper case, downcase it."
      (interactive "*")
      (let ((case-fold-search nil))
          (skip-syntax-backward "w")
           ((looking-at-p "[[:lower:]]+")
            (capitalize-word 1))
           ((looking-at-p "[[:upper:]][[:lower:]]+")
            (upcase-word 1))
           ((looking-at-p "[[:upper:]]+")
            (downcase-word 1))
            (downcase-word 1))))))