Sorry, I should read more carefully. I consider a move from unstable to stable a "downgrade" so assumed it was a move to 64bit.
If you put the stable entries into sources.list then you can explicitly ask for a stable version of something such as
apt-get install xserver-xfree86/stable
but this does not resolve dependancies so other packages are not automatically downgraded with it, packages are set to be removed if they coflict.
Have a look at the following script
apt-get install --reinstall `dpkg --get-selections | sed --expression="s/\(\S*\)\(.*\)/\1\/stable/"`
It should get the list of all installed packages and then request the reinstallation of the stable versions of everything. However since not everything has a corresponding stable version eg xserver-xorg, cpp-4.0, another way would be to save the list and remove all the entries that have no stable version and uninstall/downgrade those packages manually
dpkg --get-selections | sed --expression="s/\(\S*\)\(.*\)/\1\/stable/" > package_list
... edit file...
apt-get install --reinstall `cat package_list`
Note that downgrading the xserver from xserver-xorg to xserver-xfree86 in stable may need some re-tweaking to get x working again.
I hope thats helpful