Actually guys, the whatsapp uses the phone number you register it with as a User name.
So, no matter what device you use to "log-in" your whatsapp with, it only depends on the phone no.(user name) which you are gonna use.
Username = Phone No.
Thus, if you have whatsapp already installed in your existing device, you only need to change your "username" you use to "log-in", and it will work.
If you are changing a device, you would need to purchase whatsapp on the new device, and you can "log in" with your existing "user name"
Whatsapp only allows one "log-in" on one device at any one time. Thus, if you changes a device, the previous device would not be able to receive any further message.