Number posrtability means: You can change to another service provider.
You are asking: "I want to change number...
No. You can not change the existing number, Number remains same. but the service provider changes. and once you change from A to B, you have to be with B, for minimum period of 90 days. approach nearest 'store of your service provider' and they will guide you. As on now good services are given by 'Vodafone customer care division.