Best answer by Mats
Porting your prepaid number to postpaid should close your prepaid account. This should stop any auto top ups. I would think you still should have access to your prepaid account though and see that it is closed.
I also don't get the reps comments about any outstanding payments with prepaid. It's prepaid so there can't be any outstanding balance owed on a closed account. If anything you might have a positive balance in your account.
Yes, watch for charges on your card but I don't think that will happen.