You can use the Change Number feature to change the WhatApp number and chat with the same phone on the same phone. Just want to have your phone on it._
- Insert a new SIM phone and start your WhatsApp Account.
- Click on the Account Options in Settings. Then two boxes appear, enter the old phone number in the box above and enter the new number in the box below.
- Click on the option which will appear next to it.
- Revert to your new number. Now you can use Whatsapp from the new number.
If you have a new phone, you will have to back up WhatsApp App in the previous phone and then it will have to be restored in a new phone.