I bought the hero edition and I want to get the higher tiers while it is on sale. Any chance I can pay the difference to upgrade to higher tiers and also to purchase whitemarch+ the pass for part 2 if I already bought part 1?

Currently, there is no upgrade option to the higher tiers, so you'll be stuck with the Hero Edition till GOG decides to implement the promised (ever since the game's release date) upgrade options. As for White March, if you decided to purchase the first part instead of the season pass, then you'll have to wait till the second part is released (sometime during Winter) to buy it as well.

GOG has been rather sloppy with this, I've been waiting to upgrade my game for months. Would have loved to buy this during the sale while I had money to burn.

Cheers. I will take it as a sign that I am not meant to get the higher tier. Looking at the extras doesn't seem to justify the price. Now I regret a little bit that I didn't held off purchasing white march til I got enough cash to grab the pass. Anyway, the option to upgrade would be very welcome