Here is a post where Kris writes about the 08 differences
I could be wrong, but I think the 05-06 frames are the same as the 07. The big difference is a switch from a KH-Onza hub to the ISIS hub with moment cranks, used on the 07 and 08’s.
I think the only frame difference is the stronger seat tube on the 08’s. So if you can find a good deal on an 07 don’t be afraid to snap it up. The 05’s are less desirable, because of the non ISIS crank set up will mean you can’t swap the cranks with other uni’s. I have an 05 KH 24, and an 07 KH 20. The quality of the frames seems the same, broken seat tubes were very rare, but KH is kinda a perfectionist and did a bit better there for 08, but I wouldn’t worry about the earlier frames.