I’ve given this some thought. It could be done with a soft fork.

A fair way to do it would be to have a rolling block-height before which UTXOs become unspendable. A simple implementation would be something like MAX_BLOCK_AGE=900000, which would mean that any UTXO from more than 900000 blocks ago would be invalid. This implementation would move the wall forward at a rate of 1 block per block mined. That is, a window of 900000 blocks would always be valid.

But you could have the wall move forward more slowly, like at a rate of one block per every X blocks mined. This would create a lengthening window.

In any case, such a soft fork would be incredibly contentions because it violates the your-keys-your-coins doctrine. It may be more popular with the community to simply have old abandoned coins act as bounty for quantum pirates.