This generally depends the upon the operator to operator. Generally for operators postpaid customers are real wealth to them as they are source of fixed revenue to them , so if we genrally approach to them in order to move from postpad to prepaid they generally try that customer will not move to from postpaid to prepaid.
But technically from postpaid to prepaid the operators generally activate the sim in 15 min as everthing is already programmed at the backend and they just need to enter the number. But in real scenarios they take 24 to 48 hours due to lack of sync between there teams and avaibility of technical teams.
