Do this by using the "download only" flag in apt-get
First, change your sources.list file to point to the Karmic repositories.
Next, do a "download only" upgrade.
sudo apt-get -d upgrade
At the end of the process, instead of stepping to the "Installing packages" stage you will see a message "Download complete and in download only mode"
Finally, do the same for a distribution upgrade
sudo apt-get -d dist-upgrade
If you have more than one PC on your network to be upgraded, you probably want to use apt-cacher so that you only need to download all those packages once.
Come the time to do the upgrade proper, repeat these steps (simply to update any packages that have changed since you downloaded them into your cache).
Then, when you do the upgrade for real, it will skip past the normally lengthy download stage and straight to the "installing packages" phase while all the other plebs are fighting over bandwidth trying to download packages on the release day!