Ethereum Interview Question: How Do Protocol Upgrades Impact Deployed Smart Contracts?

How do Ethereum protocol upgrades, like new EIPs or hard forks, affect smart contracts that are already deployed on the network?

In a recent Solidity interview, I was asked this and realized I’m not sure about the specifics. I’ve worked on DeFi and NFT projects, but I haven’t had to handle protocol upgrades directly.

Do live contracts keep running as before, or do developers need to take action when Ethereum changes things like opcodes or gas costs?

If you’ve faced this during an upgrade, how did you deal with it?

Any advice or real-world examples would help.