Applications sitting behind a proxy wouldn't need to be updated. The core protocol semantics of HTTP are relatively unchanged between HTTP/1.1, HTTP/2, and HTTP/3.

[1] https://github.com/cloudflare/quiche

[2] https://www.nginx.com/blog/introducing-technology-preview-ng...

The high-level flow is pretty much:

1) Set up client connection/negotiate stuff (TLS, alpn, NPN, blah blah) 2) Process requests from that client connection 2.1) Decode request from client connection 2.2) Manipulate request (add/remove headers, ...) 2.3) Send request to server 2.3.1) If necessary, create a new connection to server (TLS, alpn, NPN, ...) 2.3.2) Encode request to server connection 2.4) Decode response from server connection 2.5) Manipulate response (add/remove headers, ...) 2.6) Encode response to client connection

You can talk totally different protocols from Internet-side client to the proxy, and from the proxy to the server - and multiple layers of proxies in between if you like. From an app point of view, there's essentially no difference. If you want to for some reason, you can use various headers to do attempt to indicate to the client to upgrade/downgrade to particular protocols, but most apps won't care about that.