In the first post you have seen the registration process using your own mobile number.
But in case, if you want to change your mobile number(because of any reason).
Then no problem you can continue with whatsapp using your new mobile number.
All you need is follow these steps:
1. Switch OFF your mobile,
2. Remove the registered SIM card and insert a new SIM card.
3. Turn ON the mobile.
4. Launch the application.
5. Application will work as before(connected to WiFi/Mobile Data connectivity).
All the people who are in your whatapp contact list are able to see your old mobile number only.
And if any one send you a message,you will able receive it as usual.
This scenario will work fine until you uninstall and install the app.
In case,if you have uninstalled the whatsapp application,
Then the following procedure will continue…
1. ReInstall the application.
2. Launch the application,again the verification process popup appears.
3. Enter the new mobile number of the inserted SIM and tap on “OK” button.
4. Verification process will be completed in 10-15 secs of 10 minutes total time.
5. Your mobile will get register with the new mobile number.
But the removed SIM will get unregistered which means that no mobile is using the number and can be used again.
Understanding WhatsApp 