Whilst abroad, I've used foreign sim cards in my phone and all Whatsapp does is prompt to keep the existing number or change to new number.

But how do you tell people to add you on WhatsApp? I don't even know my phone number. Phone numbers are a poor replacement for email addresses or usernames.

Edit: not to mention the security implications of having authentication tied to one's phone number. Any telecom employee, foreign government agent, or skilled social engineer can impersonate one on WhatsApp reasonably easily.