Upgrade from Kubuntu 9.10 to Ubuntu 10.04

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.

Personally I would always do a fresh install—for any OS, not just Ubuntu. Upgrades are always messy and can leave behind a lot of unwanted packages and conflicting config. Ubuntu has been getting better at not breaking on dist upgrades, but nothing is certain. If you're switching major DEs anyway, it's not as if you really need to keep your existing set of installed packages.

Backup your home directory and any other places you've put documents, of course (ideally, keep them on a separate partition).

I had a number of issues when upgrading from 9.10 to 10.04, mainly because of things I did myself. I had a large number of programs that were from PPAs or testing versions that seemed to give me grief (e.g., Google Earth, Firefox) after the upgrade. My suggestion as well (as @bobince mentioned) would be to back up your data to an external hard drive, wipe your original hard drive and set up a separate partition for Ubuntu and one for your personal data. Going forward, whenever you want to upgrade or if you manage to screw up your install somehow (which I have done on a few occasions) you can always just use the nuclear option to reinstall the whole thing without any data loss. This seems to break it down fairly simply, and can be done as you are reinstalling.