Development has been going on for about two years now, and our Art Lead has realized that a lot of the earlier work -- models and textures primarily -- are beginning to show their age.
So over the past month or two, S:44's art devs have been "upgrading" a lot of the art in the game. Some units are getting a simple texture touch-up to bring them up to par, while others like the entire Sherman series of tanks have been completely remodeled and retextured.
Check out the Spring: 1944 images section here on ModDB to see screenshots and renders of these new improvements.