When an intrinsically hasan report becomes sahih li-ghayrihi through corroboration, the original narrator does not suddenly acquire full precision. The first chain may still be judged hasan on its own; the report assessed through all relevant routes attains a stronger overall classification.
For example, chain A contains an upright narrator of somewhat lighter precision and otherwise meets the conditions for intrinsic hasan. Chain B supplies a credible, genuinely separate corroborating transmission. If the corroboration is sufficient, the report considered as a whole may become sahih li-ghayrihi; the biographical grade of A's transmitter has not changed.
Distinction: Assessing individual isnads and assessing the cumulative evidence for a report are related but different tasks.