Reinstalling/Downgrading Ubuntu

[NoOp]

Having a Karl moment (no offense intended Karl :-) and decided to test
how best to backdown to hardy from jaunty on one of my test systems
while preserving the /home folders. I failed as I tried to accomplish
this from the hardy alternate CD & just ended up reinstalling over the
drive.

The question of reinstalling/downgrading seems to come up every few
months, so I'm offering to play guinea pig on a test system for
suggested methods. So if you have a favorite/workable solution I'd like
to hear about it. In the end, I'd like to test & document a
reinstall/downgrade Ubuntu while preserving /home for Dummies set of
instructions. Note: yes I know that I can do this with a separate /home
partition, but in the end many/most users don't do this upon
installation (nor are there easy prompts in the installers to do the
same), so I'd like to keep to the 'preserve /home' theme.

There are several blueprints & ideas on how to accomplish this:
https://blueprints.launchpad.net/ubuntu/+spec/ubiquity-preserve-home
https://wiki.ubuntu.com/UbiquityPreserveHome
http://brainstorm.ubuntu.com/idea/14021/
[Alternate CD: reinstall Ubuntu over existing partition while preserving
/home ]
http://ubuntutip.googlepages.com/reinstall
[How to reinstall Ubuntu]
http://brainstorm.ubuntu.com/idea/5390/
[Offer to create a separate /home partition and use existing ones]

I don't mind testing the above and any other solutions folks have to
offer (it's a slow week in client land). I'd very much like to figure
out how to easily do this with the Alternate CD(s).

Note: Only standard Gnome Ubuntu liveCD & Alternate CD suggestions please.

Along these same lines, sometime back there was a thread "apt-get system
reinstallation" whereby Andy Baxter wrote up a nice howto
https://help.ubuntu.com/community/ReinstallingSamePackages
Note to Andy: I ran across this in my search on how to reinstall
existing packages & this uses Synaptic and seems to be a very easy way
to accomplish the same task:
<http://www.ubuntugeek.com/howto-reinstall-all-of-currently-installed-packages-in-fresh-ubuntu-install.html>