WARNING! This tutorial can broke your system, so no quarantee about working.
Found from https://askubuntu.com/questions/5018/is-it-possible-to-upgrade-from-a-32bit-to-a-64bit-installation
It worked in my Debian this way:
dpkg --add-architecture amd64
apt-get update
apt-get install linux-image-amd64:amd64
reboot
apt-get clean
apt-get autoremove
Old Kernel broke apt, so I needed to remove it the next way:
mv /var/lib/dpkg/info/PAQUET.* /tmp/
dpkg –remove –force-remove-reinstreq PAQUET
Manually remove a broken package
And after rebooting
su -l
apt-get -y install dpkg:amd64 tar:amd64 apt:amd64 libacl1:amd64 libattr1:amd64 aptitude:amd64 synaptic:amd64 dpkg-dev:amd64 debsig-verify:amd64 ncompress:amd64 tar-scripts:amd64
apt-get autoremove
Some programs can still be 32-bit (like browsers), so it is needed to install 64-bit versions.