From the preview:
As part of an ongoing process of improving the game, several models are getting an overhaul. There are a few sculpts and concepts that I am not ecstatic over and if I don't love it, how can I expect you to? The first revision is for the Thonian Strike Guardian. The old concept was nice but a little too bulky for a faction and a unit that is built on mobility. The nice thing about working digitally is that no work is ever really lost or wasted. The old version of the Guardian will show up at some point, but not in the current slot.