Re: case conversion by replace-match

[Richard Stallman]

    However, when foo is run in a buffer containing the string "=FC",
    this string will be replaced with an uppercase umlaut-U.

This is a feature: when replacing an uppercase word, replace-match
converts the replacement to uppercase.  If you don't want that feature,
pass t for the second argument to replace-match.

    PS In mime-encoded mails "=FC" represents a lowercase umlaut-u.

replace-match only cares that the letters are upper case.
It does not know you intend them to stand for something else.

By the way, perhaps you want to use mail-unquote-printable-region
to do this decoding.