Comment by godelski

You'll get the opportunity to change the pin.

There's a balance they want to strike. You can't assume phone numbers are unique to a person across time. So they need to be able to expire when someone stops using a number.

But again, acting on the other side also gets a notification in the chat stating that the security number has changed. The new person doesn't have the signal chat history. So if you're talking about sensitive things then it's a strong indication you should reverify their identity. Not practical for every day users but that's also not a typical threat scenario