The UI makes you believe that is the case, but people that still have your old contact will send messages to your old number and you will not receive those messages. See also their FAQ on the website [1].

    To stay connected with your contacts, we recommend you inform them about your new phone number before changing phone numbers and ask them to make any necessary changes in their phones’ address books.

I ran into this problem when I changed my number while temporarily staying abroad. After switching back to my old SIM I suddenly got a bunch of messages that I had missed from people who messaged my old phone number.

[1] https://faq.whatsapp.com/general/account-and-profile/about-c...