I replaced the pads of my HD800 some months ago and did not hear any differences. I don't think pads make any significant change on the HD800. Totally different from pads rolling on my HE-4 where sound changed pretty drastic.
as for difference between old vs new, long time ago I brought my HD800 SN02xxxx to the Sennheiser showroom where they had another HD800 SN13xxx connected to a HDVD800.
No difference I could notice. Granted, the testing environment wasn't optimal but hey what else could I do.
Just sharing experience.
Interesting. Just out of curiosity Rozenberg how old would you say the SN02xxxx series is ? or to put it another way, do you remember which year you bought your HD800 ? The reason I'm asking is I'm considering buying a used pair with SN 021300. I'm not sure how old they would be. Thanks.