I bought some about 2 months ago and had the part number when I called Sennseiser. Don't remember the part# but it's easy to find on google. They are $12 per side. I didn't think the changes were worth it IMO.
If you're weird like me you'll compare the differences. Try keeping on the HD-600 grills for one week and then switch back to the HD-580 grills the next.
I'll give it a shot.. I've had the HD600 grills my 580 practically since I got them. I've also done the APSound mod (highly recommended, IME) & have a custom silver cable for it. With the HD600 grills, APSound mod, silver cable, and a good neutral, transparent amp (i.e. any of the ones in my sig/profile).. I would best describe the sound as a baby "HE-500."
The HD580/ loves tubes, too.. running it from the TD-11's tube out (with a budget Electro Harmonix tube that's slightly U-shaped in fq curve, very detailed, and smooth) makes for a very fun, immersive sound.