Can I upgrade from Kontakt 6 to 8 and get sale upgrade price
Yes you ought to qualify for the update from K6 to K8 , provided that the Kontakt 6 that you refer to is the full version Kontakt and not player version.
When you are logged in then check the Kontakt 8 pricing page. However the best deal usually is always getting the Komplete (15) Standard bundle unless of course that you are not interested in all the other content, :
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Compare Komplete 15 bundles
Qualifying Base Products for Komplete Updates and Upgrades
