[agrover]

The IETF QUIC working is well-aware of this, and is attempting to save design room for future QUIC versions as much as possible. The "QUIC invariants" spec documents everything that is guaranteed not to change, but other than than everything in a future QUICv2 could be updated (e.g. tls version, features, large parts of packet header layout).

[yardstick]

I fear that unless they are already actively using different values for those updatable values that middleboxes will implement a de facto version of QUIC that “just works” with what is out now, but no regard to gracefully handling forwards-compatibility. Example is the SSL version field which has become static because many implementations didn’t handle an unknown value gracefully.