Freeze them? Then we've just built the thing Bitcoin was made to avoid: a committee deciding whose coins still count. Let them sit? Then the first person with a working quantum computer gets roughly a million of Satoshi's coins and the best payday in history.
My vote: a long, loud, boring migration. Ship the post-quantum address types now, give everyone years of warning, and make moving coins cheap and easy. After that, coins that never move are the owner's choice, the same as they always were. Bitcoin doesn't make anyone keep their keys safe; it never has. A quantum computer is just a very fast thief, and the defence is the same as always: move to the newer, safer kind of address while you can.
The harder question is whether the network can agree on anything in time. Coinbase speaking up helps, since they're sitting on about 5% of the supply. Nobody moves faster than an exchange that's worried about its customers' coins.
