> Doesn't this kind of thing completely belie the usual claim that it's free from central control?

It's consensus based, and the core ETH developers have a strong following. There likely will be (and probably already are) forks of ETH that keep proof-of-work, but they'll likely wither off if they don't have a large enough backing behind them.

> I don't think this is the first time I've read of a substantial change being made to a cryptocurrency. Doesn't this kind of thing completely belie the usual claim that it's free from central control?

As I understand it, what actually happens is it 'forks'. Like open source software - anyone free to use it differently or change it dramatically, but most people will stick with the branch maintained most actively or by the original maintainers.

> [There being another coin to mine] doesn't seem absurd on the face of it. Is there a reason to think this won't happen?

There are already loads, so I suppose no, of course it will/already has. The trouble is it needs a high enough valuation (with volume) to support the sunk cost of hardware to anywhere near the extent that Ethereum has for them.

> Doesn’t this kind of thing completely belie the usual claim that it’s free from central control?

Why would something changing mean it’s centrally controlled? Bitcoin has also undergone changes (although not as big as this one) and it that doesn’t mean it’s not decentralized.

This is not the first time substantial changes have been to crypto currencies (etherium or otherwise.) One recent change to bitcoin (I think the lightning network) was controversial enough to result in a fork (bitcoin cash.) There are mechanisms for this sort of thing.