bi monthly based Mail/News backup scheme

[reader at newsguy.com]

I'm looking for a way to accomplish this goal without too much regular
intervention.

I want to backup.. but no thats not really the word I need here.  It
is a backup but also a continuing non-deleting collection of what
comes on the spools over time.  So the backup will never look like or
mirror the src, except on the first run. 

It is NNTP and Mail spools (in one numbered file per message or post)
I'm working with, hundreds of thousands of them.

The end result would be (after 12 months) new top level directory started
every 4th month and no overlap where the changeover occurs.

    jan-feb-mar-apr/  may-jun-jul-aug/  sep-oct-nov-dec/

So the problem as I see it is that the src spools continue to accrue
on the new end and expire on the old with about 5000 message que or
holding period (works out to close to 1 mnth).

So when a changover occurs, since the new top level heirarchy has
nothing for rsync to find as `uptodate', it copies over whatever is on
the src spool which will include a massive amount of overlap.

I guess what I'd like to do is redirect the actual destination but
make rsync look at the old destination to determine what is uptodate
for some period.

Its a given that rsync has a wonderfully flexible set of flags and
switches. Still, near as I can tell what I described is not possible,
using only rsync switches.

Sooo, I'm wondering if anyone has devised a similar scheme with some
sort of home scripted trickery to make rsync do what I described.