Might just need to give your device some time to resync to get the names back after an update. I have seen this before when I was a regular Whatsapp user after updates. Try waiting a day or so and THEN be alarmed if you still are just a phone number.
Whatsapp has changed the appearance of contacts in notifications on purpose. Since the update you will see the contact appear just like you have saved it in your Contacts.
Their self entered user name will not be used in notifications anymore.
This is especially a real appreciated change because some people tend to have usernames like:
Username