I just did my conversion from supplementary to principal line yesterday.
All details are based on my experience and it seems that different maxis centers have different operational policies. (Another long story which involved running through 2 maxis centers, 3 times...)
The original principal owner has to clear the ENTIRE outstanding balance before the supplementary line can be taken out.
Then, there will also be a RM50 processing fee for the conversion charged to the original principal holder.
After that, depending on the plan chosen by the new principal owner, advance payment is required.
Just sharing so everyone can be prepared and don't get BILLSHOCK + FEESHOCK while doing the conversion.
Side note: I don't understand the processing fee, RM50 seems a bit much but I paid anyway. The lure of the iphone.......