There is trick to do this. Only can do when you wanna MNP to another telco.

Lets say your from Celcom postpaid wanna go to Hotlink Prepaid, just MNP out of Celcom and get to Hotlink prepaid.

You don't need to go to the ones you're porting out for any telco. Just make sure your bill got no outstanding amount, and you're no longer bound to contract. Go to the ones you'd like to port, they'll give you a sim that you'll use after the porting is done. In between, your previous telco may call to verify.

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Added on December 13, 2012, 4:19 am
Well, for digi supplementary line, actually you can just straight port into Celcom's prepaid sim firsthand. That may ease up the process a bit, since porting between telcos takes time, days even. Once you get it ready on Celcom prepaid, it won't take much time to migrate into First Prime for example.

Btw, if you're only free on 14th and plan on doing everything in one day, above holds true (changing digi supplementary to digi prepaid), since porting from prepaid should be less hassle. Still, porting takes time.