After you migrate your users to an OAuth connection, their connection with your institution will be placed in an IMPAIRED connection state. This is the same connection status that users receive when connections break from website changes, changes to the FI, and more. The user will see a connection status notification prompting them to update their credentials.
You can expect an initial call volume increase when the users are first prompted to connect via OAuth. Users may be looking for reassurances that the new call for credentials is secure.
To learn more about the user experience after clicking "Update Credentials" see the article on user experience changes.