I own the normal version of PoE 2 and would like to upgrade to the obsidian edition. At the moment this means purchasing the game again, while for new players the difference between normal and obsidian edition is 8 € ...

Gog is good about giving credits--usually automatically. If you buy the Obsidian edition you *should* get an immediate credit for your standard edition purchase. Try putting it in your cart just to see if you get an automatic credit. Or, drop Gog sales an email....