Hi Regan
There is some sound advice here, and most importantly it is that it can be hugely variable from sweating in the sun to freezing in the snow and rain.
Last year I managed the entire race in nothing more than a t shirt but then I don't feel the cold. It was cool/cold at the start and during the dark hours (that's a reference to the time of day, not an emotional state!). Friends of mine were rugged up the entire way.
So you need to be flexible, and make a specific call nearer the time. In previous years I have used the same plan as you, which was a change of kit at the 54km checkpoint, and then I had an extra layer for the final section as it will probably be dark and colder, and given that you go deep in the valley near the waterfalls, it feels a lot colder with the dampness in the air.
Last year I could have done with another layer on that last section and left the final CP shivering. The solution was to generate heat by running harder - so a win-win solution!! A really good way to motivate yourself when you are tired, just don't take it too far and get hypothermia.
Good luck
Andrew
www.ultrarunning.com.au