Thank you for reaching out to the Zoom Developer Forum. If a user changes their display name in a meeting, there is not a Webhook event that’s triggered. That means there is no way to correlate the old and the new user. If you would like this feature to be considered for a future release, I recommend posting in the #feature-requests category.
In the meantime, if you want to ensure that you can rely on a userId, you can require authentication for the meeting.
Let me know if that helps.