How Did Ethereum Successfully Transition to Proof-of-Stake Through The Merge?

I have been wondering for quite sometime now and I still couldn't get the answer. How is the transition possible? Aren't Blockchain something that nobody can change when it started? If Miners insist using the same POW chain then the transition wouldn't happen right?… Then how can it stop people from mining using GPU? It is like changing gearbox while the engine is running… Can anyone help me with this? Thank you.