We will have a quantum resistant upgrade. People will transfer their coins to quantum resistant addresses, but a lot of coins (million+) won't be moved to these secure addresses as they are "lost" coins and nobody can move them.
Which fork will you use post quantum?
a) the fork with all Bitcoins where quantum computer owners will get the "lost" coins and will be the new owners.
b) the fork where "lost" coins are burnt and can't have new owners.