Okay so you saying miners can simply change their pool to another country by simply changing the settings in the mining software/interface and that will cause china to lose some hashrate?
Miners can simply disconnect from a certain pool server and either connect to another or mine solo. This would reduce the potential hashrate that a pool can provide
Do the miners have to do this before a potential takeover by the chinese state or can it be done afterwards or too late by then?
All th mining hardware which generates the hashrate is decentralized, so the miners can always switch. There is no situation where they would be stuck in a particular pool.