They're very different.
On the first-series chairs, the legs are concave and tapered, so the chair overhangs its narrow footprint.

The sides of the later chairs are like Senator Larry Craig's self-characterization: Straight, with a wide stance.

Here are some photos of a first-series chair:

And...
This is obvious, but I guess I should mention it anyway: The first-series F598 has a wide groove between the seat and the back (hence the name "Groovy Chair"), but there's no such groove on the redesigned F598.

The original chair's wide groove emphasized the fact that it was composed of two parallel undulating ribbons; it was clear that the chair was a close relative of Paulin's beautiful Ribbon.

In the redesigned chair, though, the seat and back are pressed right up against each other, so they no longer look like two dramatic floating ribbons. Instead, they appear to be just one molded lump, with a seam on each side and an unnecessary smile-shaped cutout in the middle. In my opinion, it's a significantly weaker design.

Thanks so much for your help!
Thanks so much for your help!