It seems that users who already own articy:draft on Steam cannot use their 35% discount coupon to upgrade. This is, of course, not intended. You should be able to combine the 15% release discount with the 35% upgrade discount to have an overall discount of ~45%. We’re currently doing our best to solve this issue together with Steam. We will publish another announcement as soon as the problem is solved. We are very sorry for any trouble this causes!
Please let us know in the comments if the discount coupon works for you.
EDIT: Steam has fixed the problem and the correct upgrade discount should now be applied. Thanks a lot for your patience!
NOTE: If you don't see the correct discount coupon, please restart your steam client completely. This should solve the issue.