My recommendation to change your system to 64 bit:
1. Get a list of your installed packages and save it to a file:
dpkg --get-selections > mypackages
2. IMPORTANT! Go through the file and change all occurrences of i386 and i686 to amd64.
3. Save the file to an external medium, or to your /home partition.
4. As always before making major changes, backup your important data (/home-partition), and may be the configuration files you want to use later again from /etc.
5. Do a minimal install of Debian. Minimal means, de-select all tasks (Desktop, Laptop, Standard, etc.) when it comes to software configuration.
6. After the install comes the part to do a new install of all your packages:
dpkg --set-selections < mypackages
Now all you packages will be installed, this will take some time and will bring up some configuration dialogs.
7. Reboot to your new old system, restore your config-files if necessary and have fun.