1) Backup your /etc and /home folders (I have /home as its own partition, you may need more folders if you have any custom folders… or /var/www if you have Apache, maybe backup databases if you had any). You may use for example fsarchiver.
dpkg --get-selections > installed-software to save the list of installed software, change content of exported package list, replace word “deinstall” to “installed“, backup the file “installed-software“.
3) Install the x64 version over existing system without formating volume, create the same users. Restore the /etc and /home folders and the “installed-software” file.
dpkg --set-selections < installed-software, then run
apt-get dselect-upgrade to install the previously installed software.