How to root phone without losing WhatsApp account?
If you are planning on using the same phone again, you can simply uninstall whatsapp and reinstall it after you add the new number. Whatsapp FAQ says that you will be given an option to restore all the data at the reinstall process. They specifically ask to install the app from their website rather than downloading from Google Play. (I couldn't find any reason for this).

If you can use the previous number, you will not lose your account or the data. Simply login to the app again. 

On a side note, it is highly advisable to backup your chat history to your SD card as explained in this answer before you attempt anything (rooting, uninstall, etc.)