Dont do this at work!

http://wiki.debian.org/Multiarch/Implementation

http://users.digitalkingdom.org/~rlpowell/hobbies/debian_arch_up/

While not recommended, it is possible to "upgrade" from i386 to amd64.

First you need an amd64 kernel running. This should be no problem, e.g:
 sudo apt-get install linux-image:amd64
 sudo apt-get install gcc-multilib
 sudo update-grub

You could try it out in a sandbox:
sudo cdebootstrap --arch=i386 --variant=buildd wheezy wheezy-i386
sudo chroot wheezy-i386

You need the base system of amd64 (the debs):
 cdebootstrap -d --arch=amd64 --variant=buildd wheezy debs-amd64
 cd debs-amd64
 mv var/cache/bootstrap/* .

Now the installation begins:
 dpkg --force-depends --force-architecture --force-overwrite -i libc6-i386_2.5-9_amd64.deb
 ln -sf /emul/ia32-linux/lib/ld-linux.so.2 /lib/ld-linux.so.2

 -> here my sandbox was f** up

So I guess its easier and better to reinstall the base system and
install the same list of packages you got with "dpkg --get-selections".