So my plan is ending on 30 March and I would like to switch to a prepaid plan after that. What would be the best time to do that? I saw somewhere it can take up to 48 hours to go into effect, so should I start the process on the 28th?
Yes you are correct, a switch between a billed service and a Prepaid service takes about 48 hours.
I would contact Mobile General Enquiries & Billing on 133 937 on the 30th of March, after the switch you'll receive one final bill for charges up until switch over occurs.