downgrading from unstable to testing

[Matthew Weier O'Phinney <weierophinney@griffdog.net>]

I've been tracking unstable for about 9 months now, and for the most
part I've been just fine. However, in the last two months, I've had a
number of issues -- the libpng2 -> libpng3 migration, new perl version,
etc -- that have made it difficult for me to continue _using_ my
computer while remaining on the "bleeding edge". I know what tools I
need, and I know that they're available in sarge (and if they aren't,
I've probably compiled 'em by hand anyway!), so my big question of the
day is: how do I downgrade from unstable to testing? I know I can
"simply" change my apt.sources list to point at testing, but since most
of the software I have installed is of a newer version than sarge, won't
it simply keep telling me I already have the latest version?

Any pointers -- to the correct manuals or otherwise -- would be greatly
appreciated!

--Matthew