People code bitcoin clients. People vote for a protocol change by using those clients instead of the old ones.
A change can not be enforced. If a significant subset of clients decides on a different path (for example stay on the old protocol), a fork of the blockchain would be the result. I guess then if you would own 1 BTC before, you'd suddenly own one "old Bitcoin" and one "new Bitcoin".
There are already clients that only download parts of the blockchain, trading storage and bandwidth for reduced security - for example the mobile clients, or Multibit (afaik). I suppose they rely on trusted nodes (not sure).
But suppose SHA256 would be broken and the "vote" would be to change the mining protocol to scrypt. Why wouldn't the nodes and miners change? It's not so different from updating your web server when there has been a security bug. Some people forget to update their web servers and get hacked. The next time, maybe they won't forget to update. Maybe if you installed your bitcoin software with a package manager, it will be automatically updated.
It would be interesting if there was an attempt to enforce "tainted coins" (forcing clients to reject bitcoins that are known to have been through criminal hands). I suppose governments could require businesses to only accept btc that are not tainted. I honestly don't know how things would play out - maybe the community would addopt it, maybe not, or maybe there would be a fork...
All just from my limited understanding, please correct me if necessary.
A change can not be enforced. If a significant subset of clients decides on a different path (for example stay on the old protocol), a fork of the blockchain would be the result. I guess then if you would own 1 BTC before, you'd suddenly own one "old Bitcoin" and one "new Bitcoin".
There are already clients that only download parts of the blockchain, trading storage and bandwidth for reduced security - for example the mobile clients, or Multibit (afaik). I suppose they rely on trusted nodes (not sure).
But suppose SHA256 would be broken and the "vote" would be to change the mining protocol to scrypt. Why wouldn't the nodes and miners change? It's not so different from updating your web server when there has been a security bug. Some people forget to update their web servers and get hacked. The next time, maybe they won't forget to update. Maybe if you installed your bitcoin software with a package manager, it will be automatically updated.
It would be interesting if there was an attempt to enforce "tainted coins" (forcing clients to reject bitcoins that are known to have been through criminal hands). I suppose governments could require businesses to only accept btc that are not tainted. I honestly don't know how things would play out - maybe the community would addopt it, maybe not, or maybe there would be a fork...
All just from my limited understanding, please correct me if necessary.