Yeah, as long as you can “polyfill” or “monkeypatch” in the new JVM instructions, it’s not a huge problem: ideally you take a page from IBM’s TIMI, and you treat compilation as a form of caching: you store the JVM code on permanent storage, and compile it to the underlying hardware on first execution, saving the result until the CPU is upgraded.

Microcode obviously isn't hardware since it is program we can replace. It is firmware.