Ethereum proof of stake beacon chain has been live for 6 months now: https://beaconcha.in/
I hope I'm wrong.
What remains is to alter the clients of the PoW chain, so that it reads the PoS chain. Then instead of choosing the blocks with the most accumulated work, it chooses the blocks chosen by the PoS chain.
That's not a big change for the PoW clients, it doesn't add load to the PoS network at all, and it's all that's needed to eliminate mining.