Do I get to enjoy 6 months of Amazon Prime Video included in my plan if I upgrade my postpaid plan from Digi Postpaid 58 to Digi Postpaid 80 few months after registration?

No, the offer is given only at the point of Digi Internet Freedom registration, hence, any postpaid upgrades after registration will not be eligible for the 6 months of Amazon Prime Video courtesy of Digi.