For several reasons, I had a large number of machines running Debian GNU/Linux from the unstable distribution. Aside from having the benefit of having recent packages and a wider selection of choices of those packages, it also has a whole lot of disadvantages.
With Sarge heading slowly, but steadily, to a new release as stable, I have decided to switch over my boxes to testing. Hopefully, this will not leed to a loss of functionality, while we keep the selection of packages we have now. I also figure that the longer we wait, the more difficult it is going to be.
So, I proceeded by replacing all occurences of unstable by sarge in /etc/apt/sources.list and I created /etc/apt/preferences as explained in the above mentioned post.
X-comment: Force downgrade to testing.
Pin: release a=testing
After this, simply running # dselect update, followed by #apt-get dist-upgrade -u did all the work and I have now successfully downgraded several boxes from unstable to testing without any problems.