If the Japanese press release claimed a different tuning I would trust Anak to have mentioned it! Besides it is same model number with a mk2 designation. Axel Grell says the HD800S is very close to the original. Even if the TH900 mk2 sound is tuned differently - and reminding no one has said that is the case at all - I do not anticipate how that will assist anyone who finds the TH900 mk1 too clean, or too bassy, or too hot. Look elsewhere instead of throwing money at it?
I understand current TH900 owners want the sound to stay the same, I would do the same if I were in your shoes. Everybody wants the latest and the greatest. But Germans and the Japanese don't call something MK2 for just removable cables. They're not the Chinese who make similar stuff but charge a thousand more (HE560 -> HEX). If they call it MK2, and if they mention "newly developed driver unit", it's more than likely that there's some changes to the sound, even if the sound signature is not too different. Like Beyer T1 V2, there will probably be refinement in the sound.