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I'm not a big Ubuntu user, but the rule of thumb is when you move from the 32bit to the 64bit you should do it as a fresh install. If you don't you're more then likely to run into issues with 64 vs 32bit libraries and applications will probably behave odd. Pretty much backup your /home/username dir (hopefully you have this on its own partition, in that case during the install just choose not to format that partition), and backup any configs you have edited. Linux configs are compatible between different arches so you can use the same configs if you changed them. Ususally, I would suggest backup any configs in /etc just in case. You may not need them, but its better to have a copy and not need it then to have to figure out what you did to one all over. :)