Even if someone found a more efficient way to secure the network same like with PoW, I don't understand why we'd want that. The system works fine this way and the miners become more and more efficient, if we see this situation morally.
Wouldn't this be a hard fork?
It is. I would argue that it becomes an altcoin. The foundation of Bitcoin has mostly been left unchanged and it probably should be, barring any vulnerabilities of some sort. The game theoretic model of PoW is basically as good as it gets; giving up resources for the security of the network.
If you're finding another scheme that can secure the network in the same way, then the miners that we have today will be out of the equation. That is not going to sit well with them in the first place.
Perhaps we should change PoW, if such a great scheme comes about but the fact that you're never going to get any significant consensus with so much conflict of interest makes it hard for us to agree to a change. I wouldn't bother, PoW is fine as it is. People needs to start attacking actual climate change contributors for it or just go and penalize miners with carbon tax. That's all.