If I were to upgrade my Digi Postpaid 58 to Digi Postpaid 80 before registering for Digi Internet Freedom, will I be eligible for 'More Postpaid Lines, More Savings' offer?

Yes, as long as you upgrade to a a non-contract Digi Postpaid 80 and above plan prior to Digi Internet Freedom registration, you will be eligible for 'More Postpaid  Lines, More Savings' offer.