The API keys for third party apps still work if the developer (or Reddit) hasn’t revoked them.
I’m not sure about an API switch over, (migrating to a new version, revoking access etc), all I can remember is billing for API credentials starts rolling from 1 July and Reddit won’t start requesting payment for a month or so (citation needed)
Please correct me (with links to facts) if I’m wrong as I’m slightly out of the loop