This afternoon I migrated a WhatsApp account from an old Android phone (Sony Xperia) to a much newer one (Google Pixel 3a), keeping the same number.
As the chat history did not initially transfer across, I relaunched WhatsApp on the old device in order to create a backup I could then restore on the new device.
At that point I got the following message on the old device:
"Your phone number is no longer registered on this phone. This is likely because you registered your phone number with WhatsApp on a different phone."
And then the one button, 'VERIFY'.
I took this to mean that by clicking the button, I verified the accuracy of the message and the fact that I had indeed registered my phone number with WhatsApp on a different phone. So of course I clicked it.
It turns out it actually means 'verify (or rather, register) this old device with your WhatsApp account again'. So if you click it, and complete the steps that follow, you will no longer be able to use WhatsApp on your new device. And unfortunately you need to do this in order to create the backup.
User journey aside, the button and accompanying message could be made much less ambiguous. Perhaps something like:
"Your phone number is no longer registered on this phone. This is likely because you registered your phone number with WhatsApp on a different phone. Click on the button below to re-register your phone number with WhatsApp on this phone instead."
Followed by the word 'Register' rather than 'Verify'!