Is it possible to export my UDK 2 project to UDK 3 ? What kinds of things aren't transferable?
The answer to your question is given over here
Q: Can previous Unreal Engine 3 projects be imported?
A: Yes, users can open Unreal Engine 3 packages made with previous UDK releases in more recent UDK Betas with iOS support.
So if you have made some projects in UDK 2 it will work in udk 3 with some changes to the project file in the kismet and basically some code changes.
True, the Unreal Development Kit only exists for Unreal Engine 3.
All of them support kismet.
You can load almost all content from an older version of UDK to a newer version with the exception of certain deprecated content such as the pre-Scaleform ui solution. Other things such as the old Terrain system were replaced with newer systems (Landscape) and thus need to be converted with in-engine utilities.
Also certain other things, like material editor nodes, may have become obsolete but are still in the engine, so you may need to go through and swap out old nodes (specifically certain depth nodes) for the newer ones.
Also Shader Model 2 support was deprecated so you could see some issues with left over fallback materials.
The bigger the leap forward the more work may need to be done to upgrade.