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You have to be a Platinum Member as the offer seems to be only posted in the Members Forum.  It says that if you purchase Carrara 7 or 7 Pro or upgrade from 6 / 6 Pro that you will get a free update to Carrara 8 (same Pro or Standard version that you bought upgraded).  The extra discount apparently is on Platinum Members.  I show the Plat price for upgrading from 6 Pro to 7 Pro at $99.93 even if you bought a Platinum Membership for one month it would still be an excellent savings.  

I dont do enough to justify a Platinum account, but if the $99.93 was offered to regular members I'd buy it right now.