Does the change to Mir affect other Ubuntu derivatives like Xubuntu or Kubuntu? For example: Will Gnome 3, Xfce or KDE be ported to Mir?
It's complicated :)
No it won't is the short answer. Canonical wants to move to Mir. Their effort to continue supporting X will be minimal once Mir starts working well. X will then have to be developed and supported by the community (that's us). The story is that the Kubuntu developers are not inclined to move to Mir. So Kubuntu will either stay with X or continue to move to Wayland.
However, no one believes that the dust has settled on this. On the one hand, KDE/Kubuntu will have to start supporting both X and Wayland so that will be a heap of extra work. On the other, we will all have to see how well Mir actually works. If it isn't up to scratch to replace X in the coming months, Canonical may have to extend support for X longer. So let's just wait and see.
No, at least not in the short run. In fact, both Lubuntu's and Kubuntu's developers have announced that they won't be using Mir in their 13.10 releases. Lubuntu will not switch to Mir in 14.04 either. However, Mir's developers have been able to run Ubuntu and its derivatives on Mir using the XMir compatibility layer.
Edubuntu, Xubuntu & Ubuntu Kylin have not yet announced whether they will switch to Mir.