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If you are going back to a prior Ubuntu level, you simply need to move /home to a separate partition. Then re-install keeping that /home and the same userids.
Simply resize the current system to free up enough space and create the new partition - then "cp -a ..." will do it. All should be done from a liveCD like gparted to ensure no issues with mounted filesystems.
You will need to re-install any apps you added yourself to the base system over time.