When using Microsoft Word 2000, if you try to replace an all uppercase string (e.g. XXXX) in a document with a mixed case string (e.g. FileName), then Word helpfully makes all letters in the replacement string uppercase (e.g. FILENAME). On the other hand, if the original string had mixed case (e.g. XxXx or xXxX) or was all lowercase (e.g. xxxx), then the case in the replacement string is preserved.
If the original text is all uppercase, then the workaround is to select the Match Case checkbox in the Find and Replace dialog.
This is just a plain annoying feature. I can't think of a reason why anyone would want their replacement text to be changed based on the case of the search text.