When your phone number changes, WhatsApp asks if you want to change your number. You don't have to accept this, it will work fine on your old number. It is risky if you lose the SIM card but it works, I continued using an old number for three or four years after changing numbers and I was never forced to prove that I still was the owner of the old number.
And when your old phone number gets re-used by the telco, some random person will start getting your messages.
A friend was quite freaked out when multiple afgans started messaging him on whatsapp on their "new" phone number. Apparently it formely belonged to a member of the local afgan diaspora.
Thankfully they ware able to get a new number from the telco to deal with it.
