Programming language upgrades are not the same as blockchain upgrades. Blockchains work on consensus, so you need a majority of nodes to agree with your changes

Programming language upgrades are not the same as blockchain upgrades. Blockchains work on consensus, so you need a majority of nodes to agree with your changes to the chain or you risk a hard fork. At any rate, ETH2 proof-of-stake is already live, the last remaining piece is The Merge, which merges the existing proof-of-work chain into the already live and operational ETH2 proof-of-stake validator nodes.