Good question, and something that might eventually need attention, together with market/liquidity isolation. A question though that should I think, if it is relevant, be mostly relevant for brand new nodes that need to sync the whole chain from zero.
As stated, my proposal above is just a small first step on the hash-based quantum-resistant signatures part of things. I think we need a roadmap and people like you who know to ask the right questions to come to that roadmap.
Maybe you could look into the new node aspect in a blog post and post it in the (still fetal sized) community ?
I suppose having a separate balance that uses quantum-resistant keys would be a good first step. Those keys would be preferred after that. Whereas the legacy (non-quantum-resistant) keys would be less preferred, for a time. Then, once adequate adoption is achieved, the legacy keys would be fully discouraged.
Then, after enough time has elapsed, the legacy keys would only be used for replays, perhaps at that point, using quantum-resistant checkpoints during the replay.