If you follow their blogs etc. the fact that they didn't have an option for an upgrade to Collector's, or a discounted price if you already have purchased the game, is contrary to what they preach. Their business model is about their fan base and to not have any option for those who 'supported' the game while it was being created is disheartening/disappointing.
I understand too that there had been an announcement about an upgrade for Early Access Steam Accounts but announcing it and actually do it. Are two separate things. (Color me impressed once they have implemented it)
OK stop, Larian are a small independent developer composed of a few dozen guys so it takes time to get everything lined up and ready for launch....they don't have a PR army and hundreds of people. Have patience, there's still weeks to go before release and that's really the only time being unable to upgrade would have any meaningful impact on you.
