hike works on 1 mobile - 1 number principle. To change your mobile phone/device, you need to unlink (logout) your account from your old device first. Once done, you can use the same mobile number to register from the new device.
To unlink (logout): Go to Menu > Settings > Manage Account > Unlink Account. (For Android users, 2.5 and above : tap on options menu from your home screen > Settings > Account > Logout) Please note that by unlinking your account, you will loose all your previous chat history, as the same is stored in your handset!