The content of the old conversations is still there, but attempting to add a new prompt to the conversation results in a 400 error. New conversations work correctly. Based on a quick look, it appears that the API requests from old conversations have an empty model payload parameter (see screenshot). By comparison, the new conversations use text-davinci-002-render-paid instead.
If I add the new model parameter to an old query and resubmit it using curl it appears to work correctly.