[Exim] How to run filters on existing mail spools

After a lot of tinkering, I finally have filters working properly.
But I can't figure out how to apply them to existing mail.

Here's the situation:
The mail I want to filter is mostly sitting in /var/spool/mail/ross
and ~/mail/xxx.
Most of the filtering is done by my personal .forward file.
Running exim 3.12 on debian potato.

So exim has already processed the mail and "delivered" it; I want it
redelivered.

This question seems to be too elementary for the FAQ. I'm sure the
docs have it implicitly somewhere, but I'd appreciate a pointer.

P.S. I signed up through the e-groups interface; I hope that will
work. I've received a greeting, but no message to which I must
respond to subscribe. Also, egroups doesn't seem to offer much more
searching than your site has--that is, there doesn't seem to be an
option to search more than a month's messages at a time. I know this
is possible with other egroups.

Thanks.