If you want to keep your number active for a newly activated Sim cards simply recharge with airtime or bundles on the first 30 days. Thereafter recharge with airtime or bundles at least once every 90 days.
Buy and activate a new SIM. Once activated, dial *135*3# on the new SIM to start the SIM swap process.
The XtraTime service fee is R1,10 per R10 advanced. Whether you qualify for XtraTime will depend on your activity on the network, which includes how often you recharge. – Check on service fee amount
You can change your Prepaid Price Plan on the MTN App and on *136#.