Ubuntu – Why is Karmic (9.10) neither available from “releases” nor from “old-releases”

The best thing to do would be to first upgrade to 10.04 normally. Make sure everything is at least working the way you want it to. Then, to start the transition, install the ubuntu-desktop package in Synaptic or via the command-line. That will pull in a bunch of new applications and dependencies.

Afterwards, reboot, and you'll should have GNOME ready to use. If you don't see it, choose "Ubuntu Desktop" or "GNOME" from the login screen. You will still have a lot of KDE applications installed at this point; remove or keep whatever you see fit.

Another thing to note is that you may be asked which display manager you want to use (gdm or kdm), simply select GDM to get the GNOME display manager and sign in from there. KDM should still work fine as well and you can also continue to use that if you like.

Once you've got it all cleaned up, remove the kubuntu-desktop package to stop receiving updates to the Kubuntu distribution on future upgrades. Enjoy.

No, upgrade is only possible to the immediately following release except in the case of a LTS release, when direct upgrade from one LTS to the following is allowed. You have to first upgrade to 10.04 and then to 10.10.

The command update-manager -d is used for upgrading to a development version. Since both 10.04 and 10.10 have been released to public, you cannot upgrade to either of them using that command.

Your update-manager must be showing the button for upgrading to 10.04 when you start it normally(ie without the "-d" option). If you don't want to reinstall then use that button to upgrade to 10.04 version first and then upgrade to maverick in the same way.