Perhaps, it means to list them all in a note manually (write it down),
including the repo address. So we can install it again by searching it,
e.g via Ubuntu Software Center.

No, the commands Jim was after (finally found them!) are - to list
installed packages

dpkg --get-selections "*" > Desktop/applications

and to reinstall them

sudo apt-get update
sudo dpkg --set-selections < Desktop/applications
sudo apt-get -u dselect-upgrade

That's what I was looking for. Many thanks.

I did a quick look of the resulting Desktop/applications and was
surprised to find "sudo", "synaptic", etc. Do I really want to install
them?

No problem, happy to help :-)

The thing is, with that command, you get *all* the installed packages.
That includes all the packages in the base install plus any dependencies
installed along with the packages you installed later.

Any packages in the list that are already installed as part of the base
system should just be ignored. The thing that worries me is if you have
any packages in the list that aren't in the repos for the new version
(particularly libraries), whether or not the reinstall process would
fall down (I don't know whether it would, I've never used this way of
doing what you want, but I suspect that it might) in which case, a
little judicious manual pruning of the list might well be in order -
don't worry about deleting any dependencies for the packages you want,
dpkg should be intelligent enough to pull them down from the server for you.

In my experience, it's not perfect, but it's a /lot/ better than having
to reinstall everything from scratch.

In the end, though, I found the best thing is documentation - make a
note of what works (and what doesn't), and refer to that, and update it,
everytime you need to reinstall.

It all depends on how many outside sources you have and whether you install
things like GoogleEarth from bin. For most people it will do the trick. You
can also do this from GUI using Synaptic which is better for people with an
aversion to the commandline or if you cannot remember seldom used commands
or don't like typing paths. In Synaptic, go to File, Save Markings and check
the box at the bottom to Save full state.
The reverse afterwards is equally simple. File, Read Markings. You have to
love Linux for its flexibility. There is almost always more than one way. :)

since I've only using upgrading, I never though there is a way
like this. Thanks, I may find it useful in future :-)