Sorry..
Here's the correct definition of "replace":
Action replace
Syntax: replace :header <header> <text>
replace :body <text>
replace :body :file <file>
The "replace" action replaces a line, or lines, of text in either a
header field, or the body. In either case, the "<text>" is one or
more lines of text used to replace an existing header field or message
body. In the case of ":header", if the header field does not exist,
then it is appended at the end of the header.
The "replace :header" action replaces an existing header named <header>
with <text> or the contents of <file>. If the header doesn't already
exist, then the action is identical to "append".
The "replace :body" action replaces the current mail body with the
given
text or file contents .
If ":file <file>" is given, then the replacement text is obtained by
reading the "<file>".
For alternatives, please see the "insert" or "append" actions.
Example:
if header ["To"] :contains "ietf-mta-filters" and
not header ["X-Filed"] :contains "ietf"
{
replace :header "X-Filed" "ietf"
resend "Local-IETF-List"
}