That's a very good question, and the answer is: you can change as little or as much as you want from the original game / system.
One thing I've realized with time is that the core of a system is different from one person to the other. It's really opened to interpretation. Some people think that what makes a game Guided by Firelights is the Story Deck and the map building, while others may think it's all about the interconnected actions and triggers. And the real answer to all of this is that it depends!
I have my own opinions, but I don't want them to affect people's creation and vision. For that reason, you can change the original text/rules as much as you want, or even copy/paste it and only change a couple of words.
For example, Pandion Games is remaking Substratum (which was originally a Breathless game) and mixing Breathless with Firelights to create something new and unique.
When in doubt, go with what sounds fun!
