Pipe a whole mail box to a filter

[James Mason <james.mason@xxxxxxxxx>]

I use procmail to filter mail to different mail boxes.  I would like to
change one or two of my rules, and would like those changes to filter mail
that has allready been received. The only way I can think to do this is to
pipe each message to procmail in turn. This would be very labourous. Is
there a way to pipe the entire contents of a mailbox to procmail (or any
other filter for that matter), or am I stuck doing it by hand.

Thanks
James