Zettelkasten Notes should be immutable; I should never delete old notes and I should never modify the text of old notes.

This will allow me to see how my understanding has developed over time and revisit old understandings that might actually turn out to be more correct than my current understanding.

If I find that my understanding has changed, I should link to the old note when writing a new one and explain how my understanding has changed.

Source: Zettelkasten — How One German Scholar Was So Freakishly Productive