Stock Tires - What do you do with them after replacing?

Depends where you ride and in what conditions. I’ve had GP4000s with multiple cuts and punctures in <1000 miles riding on UK roads in winter. That does start to get expensive if you’re doing high mileage, and repairing punctures when it’s cold and wet is no fun at all. 4 Seasons work better for me as being more durable than the GP4000s and grippier and more comfortable than the Gatorskins. Switching to tubeless before next winter, hopefully running sealant will get me a bit closer to the holy grail of speed, comfort, grip and durability.