> Uwe Brauer wrote:

> You are right, I only tested for headers with a value, but now
> it works:

> (defun message-set-header (hdr val)
>   (interactive "sheader: \nsvalue: ")
>   (message-position-on-field hdr)
>   (message-beginning-of-header nil)
>   (delete-region (point) (point-at-eol))
>   (insert val) )
> ;; test:
> ;; (message-set-header "Subject" "Lara loves YOU")

> (defun message-transpose-headers (hdr1 hdr2)
>   (interactive "sheader 1: \nsheader 2: ")
>   (save-excursion
>     (let ((v1 (or (message-field-value hdr1) ""))
>           (v2 (or (message-field-value hdr2) "")) )
>       (message-set-header hdr1 v2)
>       (message-set-header hdr2 v1) )))
> ;; test:
> ;; (message-transpose-headers "To" "Subject")


For single lines addresses yes, 


To: user1@gmail.com
Cc: user2@gmail.com

Changes to 

To: user2@gmail.com
Cc: user1@gmail.com


But if you have multi line addresses, which can happen if you use bbdb
then

To: user1@gmail.com, user3@gmail.com,
    user4@gmail.com
Cc: user2@gmail.com

gives

To: user1@gmail.com, user3@gmail.com,
    user2@gmail.com
Cc: user1@gmail.com, user3@gmail.com, user4@gmail.com