2. Backup current chats by opening your whatsapp and going to Settings>>Chat Settings>>Backup Conversations.
3. Open your file explorer and rename /sdcard/WhatsApp folder to /sdcard/OGWhatsApp
4. Uninstall the official Whatsapp or clear data(I recommend uninstalling) and reinstall (again).
5. Put the new line for the whatsapp in the phone and register it with it.
6. Download OGwhatsapp from HERE
7. Register the OGWhatsApp with the old number.
8. Allow the OGWhatsApp to restore the backup data if it asks.
9. Ensure it finishes restoring the data, if not try to skip the step.
10. Send your new number a message from your old number to ensure it works at real time.
EDITED BY DRACON