If you stay on the same package....then the unused airtime will carry over as per normal. If you were to migrate to a Weekender or Talk package when doing the upgrade, then the accumulated airtime will be forfeited end of the month when the migration gets pushed trough by the system. Reason for this is that airtime can not be carried over to a non Top Up contract as the Talk and Weekender tarrifs runs on a seperate billing engine. Same should go for the data bundles active on the line.
Regarding the finance charge that is on your line currently. This is worked out as per the 24 month contract. Therefore, it will remain on your account for the 24 months as per the agreement between you and Vodacom. Once the 24 months have lapsed, the amount will be removed. Should you take another finance deal with your next upgrade 9upgrade date is 3 months before the 24 month period is over) you will be paying two finance charges untill the first one lapses.
OC