How does upgrading work?

[Cityscape]

Can I upgrade 9.04 to 9.10, what about 9.10 to 10.04? Are there any problems doing this?

I have Ubuntu 9.04, I download the VirtualBox deb package for Ubuntu 9.04 from the website. If I upgrade to 9.10 will VirtualBox be upgraded to be compatible with 9.10?

Also I've heard that Ubuntu can upgraded to Kubuntu or Xubuntu by install their packages. If in installed the Kubuntu package would it be exactly Kubuntu, or would it still have all the Gnome apps installed? What about Xubuntu?

[Slim Odds]

Can I upgrade 9.04 to 9.10, what about 9.10 to 10.04? Are there any problems doing this?

I have Ubuntu 9.04, I download the VirtualBox deb package for Ubuntu 9.04 from the website. If I upgrade to 9.10 will VirtualBox be upgraded to be compatible with 9.10?

Also I've heard that Ubuntu can upgraded to Kubuntu or Xubuntu by install their packages. If in installed the Kubuntu package would it be exactly Kubuntu, or would it still have all the Gnome apps installed? What about Xubuntu?

First three: yes, yes and sometimes.

VirtualBox: not necessarily, you might have to do that yourself.

Kubuntu: Installing kubuntu-destkop will add the KDE desktop to your system, it will not remove GNOME stuff at all.

[Cityscape]

Okay thanks. What kinds of problems could there be with upgrading?