We're talking about minor aesthetic changes here, most noticeably to the Fido, Zeus, Flash and Core Commander, but improvements nonetheless. In closeups the units look noticeably better.
Be warned that I have no idea as to whether or not they work in multiplayer mode, but I don't see why they wouldn't, as they're just models (no actual unit data is changed).

The upgraded models are available from the Evolva site, but they come individually in a collection of 25 or so zip files, and are a pain in the ass to extract, so i've bundled the whole shebang into the one zip file that rules them all, so you can save wear and tear on your mouse.
To install, extract the contents of the zip file into a folder called Objects3D inside the Spring directory.
Edit: larger pack of Evolva models put together by zwzsg for those of you too lazy to read.