By testing the previous instructions for a full crosgrade I run into trouble. Here is the results of my tests to do a full crossgrade of a minimal installation of Debian inside a VM.
First you need to install a 64bits kernel and boot with it. See my previous post on how to do it.
Second you need to do a bootstrap of crossgrading:
apt-get clean apt-get upgrade apt-get --download-only install dpkg:amd64 tar:amd64 apt:amd64 bash:amd64 dash:amd64 init:amd64 dpkg --install /var/cache/apt/archives/*_amd64.deb dpkg --install /var/cache/apt/archives/*_amd64.deb dpkg --print-architecture dpkg --print-foreign-architectures apt-get --fix-broken --allow-remove-essential install
Third do a full crossgrade:
apt-get install --allow-remove-essential $(dpkg --list | grep :i386 | awk '{print $2}' | sed -e s/:i386// )
This procedure seams to be a little fragile, but worked most of the time for me.