there are two options actually

1) submit MNP
2) submit MCMC complaint

once you receive call from retention then straight can convert from Maxis Postpaid to Hotlink Postpaid. no need to change to prepaid also.

I have changed from Maxis Postpaid to Hotlink Postpaid very recently.
I have followed their advise, which told me to change to Prepaid, use it for almost a month, then call in again to change to Hotlink Postpaid. No need to walk in to any service center. The whole process took me about 1 month plus to fully swap to Hotlink Postpaid.