Back when Pillars of Eternity was released on GOG.com there were no upgrade paths to higher tier edition - unlike on Steam. However, we have been promised this:
Source: https://www.gog.com/news/release_pillars_of_eternity Well, no such thing was ever introduced.
Now there's the new Definitive Edition which includes all extras of the Royal Edition. As I'm a big fan of Obsidian's work and I'm one of those customers who bought one of the lesser editions of the game I would have loved to upgrade to the Royal Edition at some point but we never got those upgrade options. I'm very disappointed that this situation hasn't changed with the new Definitive Edition. Again GOG users cannot upgrade to a better version of the game without buying a full version of the game again.
It's pissing some people off over at GOG and I think rightfully so. Obsidian, can you get in touch with GOG staff and work something out? This situation creates more frustration than it should be.
Edit: I'm not the only one who feels this way, just take a look at the PoE DE release thread at GOG.com: https://www.gog.com/forum/general/release_pillars_of_eternity_definitive_edition_63164/page1