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For me, it depends on whether the backstory is completely true or not. Swapping to first person (almost a monologue) permits the new story teller to lie. Holding it in third person makes it 'true...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/41356 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
For me, it depends on whether the backstory is completely true or not. Swapping to first person (almost a monologue) permits the new story teller to lie. Holding it in third person makes it 'true'. Changing narrator may not be common now, but has been used in the past (Bram Stoker, Wilkie Collins that I know of). Fundamentally, whatever works for the story you are telling.