Re: What will happen if quantum computer owners start to move the early mined coins?
quantum resistance =/= cutting edge. lamport signatures are from the 70s. 
centralized entities can quickly implement new encryption and reverse most of the damage done if QC breaks current standards. they can afford to wait. bitcoin is irreversible, and people take years to upgrade. there would be 5 million+ coins up for grabs if QC broke ECDSA today---and that's not even considering p2pkh inputs that might be stolen in-flight.
if we want to reduce that number (ie prevent many millions of coins from being stolen) we need to act sooner than later, because again---people take years to upgrade their bitcoin software. by various estimates, ECDSA could be broken within 3-7 years. that doesn't leave us much time.
centralized entities can quickly implement new encryption and reverse most of the damage done if QC breaks current standards. they can afford to wait. bitcoin is irreversible, and people take years to upgrade. there would be 5 million+ coins up for grabs if QC broke ECDSA today---and that's not even considering p2pkh inputs that might be stolen in-flight.
if we want to reduce that number (ie prevent many millions of coins from being stolen) we need to act sooner than later, because again---people take years to upgrade their bitcoin software. by various estimates, ECDSA could be broken within 3-7 years. that doesn't leave us much time.
People don't upgrade their nodes because they are compatible with the network, if there was a hardfork, you can expect them to switch rather quickly. Look at ETH or other alts, they have hard forks almost every year and they go pretty smooth.
If quantum computers became real today, the world would be in a lot of trouble - bank account, emails, websites, military communications, classified data and so on - but you don't see them proactively moving to post-quantum crypto right now, which means that the cryptographic community doesn't view as an immediate threat. And cryptographers usually retire algorithms long before they become not even fully broken, but just weakened.