My service invoice says all Tesla branded parts installed by Tesla are covered under the limited warranty for 12 months.
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My MCU purchase and FSD upgrade were labeled as Customer Pay and Customer Pre-pay so that's the coverage that seems to most reasonably apply.
It was also my assumption that if more than 12 months remained on the vehicle's limited warranty, that any upgrade parts would be covered under that, since it would be hard for them to justify not honoring the warranty on the original MCU or FSD HW they replaced.