I heard Days retcons itself at the end of the game
No, it just ensures that most of it will never be mentioned again. Basically, all the bits that serve as background/behind-the-scenes explanations to CoM and/or set up KH2 are fully canon and relevant. All of the internal stuff that's only relevant to Roxas is also canon, but [see edit history for explanation]
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"My name is Ezio Auditore da Firenze, and like my father before me, I am an accountant"
Nothing is true. Everything is tax-deductible