In order to change your network provider, you need to use the MNP service (Mobile Number Portability). Depending on the service that you are using this can be done differently. For pre-paid cards, you need to have a minimum of 90-days before you can change the provider and for post-paid cards, you must pay the bills to the new provider after you changed it.
To change the provider, you need to send an SMS to 1900 with the text : SMS ‘PORT (Your Current 10 Digit Mobile Number). After you received the message with the UPC number (Ultra Porting Code), buy a new card, complete the form and the UPC number and wait for the porting action to be registered. To be noticed that this service is only available in India.