Re: Advice needed on 10.04 LTS to 12.04 LTS upgrade.
If /home is not a separate partition, you can't save it. Because best is to format the / partition when installing the new version, and if home is only a folder in / it will get formatted too.
If /home is a separate partition, you can format / but not format /home and reuse it as /home again.
Also, I don't think you upgrade to a new release with dist-upgrade, that it only to upgrade all packages to the most recent version. To upgrade the ubuntu release you use do-release-upgrade.
In any case it's best to have a full backup, which ever choice you make.
Darko.
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Ubuntu 18.04 LTS 64bit