[exim] Subject rewriting with Exim4.6 and spamassassin 3.1

Hello there,

I have exim 4.6 and SpamAssassin 3.1 on Linux RH7.2.

I have would like to have my Subject changed if e-mail contains spam
($spam_score >5)

my configure file:
"
warn spam = mail:true
message = X-Spam_score: $spam_score\n\
X-Spam_score_int: $spam_score_int\n\
X-Spam_bar: $spam_bar

#warn message = X-New-Subject: *SpamAssassin*($spam_score) $h_Subject:
warn message = Subject: *SpamAssassin*($spam_score) $h_Subject:
spam = mail
"
Configuration like this is adding second Subject header to e-mail, insted of
rewriting existing one.

I have another e-mail antispammer and would like to have
*SpamAssassin*($spam_score) added to original e-mail subject.
To compare which antispammer caught more spam.

I have tried adding

rewrite_header subject ***SPAM***
to /etc/mail/spamassassin/local.cf

to no avail, this option does exactly nothing in my case.
I have set up user:mail (exim is working as such user) with nobody
SpamAssasin could not open /etc/mail/.spamassassin folder.

Any help, advice would be greattly appreciated.
Regards
Slawomir Orlowski