The client _must_ use the correct protocol, that's precisely the issue. An insecure request to switch to a secure protocol would just be elided by an attacker.

The client _must_ use the correct protocol, that's precisely the issue. An insecure request to switch to a secure protocol would just be elided by an attacker. If the client is coded to expect such a switch and fail if it doesn't happen, that client is at best implementing a more complicated, ad-hoc secure protocol.