I believe even the legacy addresses that are unspent with no other outgoing transactions are quantum resistant already.
The legacy P2PK addresses, even unspent with no outgoing transactions, are not quantum resistant. Satoshi knows it, but he has more than 1 million Bitcoins on P2PK addresses.
That's the reason why one should transfer the Bitcoins from old P2PK addresses or used addresses to new unused addresses to make them quantum resistant.