I think Ethereum since it's not a currency but a decentralized computer, the rules for Ethereum are different than for currency. For currency it absolutely has to be immutable because money is like memory and it can't be changed. Ethereum is like a computer running decentralized applications which may fail, these apps can be killed, closed, stopped, in order to save the computer.

So I agree with the software upgrade depending on how they do it. Ethereum is going Proof of Stake anyway so it's not like it's supposed to be like Bitcoin. Steem also reversed a hack before.