To have both installed, you can install ubuntu-dekstop for Gnome-based ubuntu, and kubuntu-desktop for Kubuntu.
If you have both installed, you can configure the default display manager with
sudo dpkg-reconfigure gdm (or kdm)
Either way you can choose whether to use Gnome or KDE at the login screen.
To remove one, you can uninstall ubuntu-desktop or kubuntu-desktop, then run
sudo apt-get autoremove
Depending on any additional packages you installed, some dangling packages may remain.
If you have a separate home partition, you can just reinstall Ubuntu/Kubuntu, formatting your root partition, without touching anything in your home partition. This will give you a clean installation, except for some config files remaining in your home partition. Of course, still make sure you have a backup of your important data.
You can't.
Mint and Ubuntu, no matter how closely related they are (or aren't, as the case might be) are still two different operating systems.
This is a bad idea.
First, you're mixing two different distributions' packages together. For various reasons, this is a bad idea:
Security.
Stability.
Conflicting updates.
If you let updates go through, you encounter the first two problems, and then you're in deep trouble if say, as an example a kernel update comes through!
Also, this makes getting an accurate bug report or stacktrace almost impossible, if not entirely impossible.
Best Answer
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-desktoppackage 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-desktoppackage to stop receiving updates to the Kubuntu distribution on future upgrades. Enjoy.