Late 2007 saw the second and final update of the series, the WACS7500. Feature-wise it was pretty much the same as the 7000, but had colour displays instead of the shonky 'sci-fi' looking blue ones of old (don't get me wrong - I liked them!) and could display album artwork while playing your music. I've had a couple of 7500 centres and they seem less well made than the earlier models, plus they're harder to do the hard disk -> SSD upgrade on. As with all the others in the line, none of the internet-based stuff works anymore.
So, not much in the way of feature upgrades, but a useful point to note is if you like the look of the 7500 stations they will work happily with an earlier centre unit. In fact, you can mix and match 700/7000/7500 centres and stations to your heart's content - any combination will work just fine.