ARC register is for changing your status as certified resident in Korea. So that request at carrier center that extend your validity from 90days to permanent.
If you do not register the ARC, after 90days your SIM card can be cancelled randomly by following Korean government rule. This is to prevent illegal immigrant.
Also, prepaid plan means pay before you are using the service and Postpaid means pay after used. The pay system is totally different and LG U+ MVNO(yy telecom) does not support postpaid plan.
AutoPay is only recommended to be used to reduce the inconvenience of charging every month, not to change to a postpaid plan.
If you want to change to the postpaid plan to keep your current number with prepaid, make an postpaid order and move to other carrier which is KT MVNO(skylife). And it also mean by need to change the SIM Card.
Please be notice both service carrier is different so that prepaid plan cannot be move to the postpaid without changing the carrier.