Now the question is if the new front pulley/diff has a different number of teeth which would affect the internal drive ratio. A good thing if they made it smaller as it would help with stock classes. That might explain the new front/rear belts included in the conversion kit.
All the extra comments suggest the idea is if you have a 2015 car, you can get the conversion kit and you will basically have two cars for different conditions. Not a bad deal and a good strategy for Yokomo as this might eek some money from people who otherwise would not upgrade to the new kit.
I think the short shock upgrade existed in the past as an option.