I dropped by at Maxis Centre Gardens again just now and they told me the same thing that they told me yesterday. On 31/7 when you collect, you can still change the plan, you can even change the phone line (to supplementary or to primary) or even register a new number to be used with the iPhone. In fact, they said everything can change except the iPhone model which must follow what was booked.
So we still have 2 weeks to decide what plan suits us best.