By efault, on a cpanel system, Spam Assassin doesn't allow the subject line rewriting.  I find it a quick short cut to be able to glace at the subject line and tell if the email is junk or not ... Here is how to enable it.  Requires root level access on the server.

First off - thanks to the exim wiki

Start off by saving your current exim config :
cp /etc/exim.conf /etc/exim.conf.bak

Open /etc/exim.conf in your editor of choice, I use vi

go to line 390, and you should see something like :

warn
condition = ${if eq {${acl_m0}}{1}{1}{0}}
spam = ${acl_m1}
log_message = "SpamAssassin as ${acl_m1} detected message as spam ($spam_score)"
add_header = X-Spam-Subject: ***SPAM*** $h_subject
add_header = X-Spam-Status: Yes, score=$spam_score
add_header = X-Spam-Score: $spam_score_int
add_header = X-Spam-Bar: $spam_bar
add_header = X-Spam-Report: $spam_report
add_header = X-Spam-Flag: YES
set acl_m2 = 1

Just before the last line we need to add a new header to the message :

add_header = X-Spam-Flag: YES
add_header = X-New-Subject: ***SPAM ($spam_score) *** $h_subject
set acl_m2 = 1

Save the file and restart exim. Now, when a message is detected as spam, it will add a header named X-New-Subject.

Now we need to edit the exim system filter, which on a cpanel system is located at :

/etc/cpanel_exim_system_filter
make a backup : cp /etc/cpanel_exim_system_filter /etc/cpanel_exim_system_filter.bak

Then open in your editor and add the following lines @ the end of the file :

if first_delivery then
if $h_X-New-Subject: is not ""
then
headers remove Subject
headers add "Subject: $rh_X-New-Subject:"
headers remove X-New-Subject
endif
# ...
endif

This first removes the existing subject, then adds a new Subject line with the spam tagging and the spam score.