Emacs: regular expression replacing to change case

Try M-x query-replace-regexp with "<\([^>]+\)>" as the search string and "<\,(downcase \1)>" as the replacement.

This should work for Emacs 22 and later, see this Steve Yegge blog post for more details on how Lisp expressions can be used in the replacement string.

For earlier versions of Emacs you could try something like this:

(defun tags-to-lower-case ()
    (goto-char (point-min))
    (while (re-search-forward "<[^>]+>" nil t)
      (replace-match (downcase (match-string 0)) t))))