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I can’t imagine it would be cost effective given that the battery pack is by far the most expensive component in the car. Certainly the best option for a new pack would be to trade the car in on a newer model. As for used packs, Nissan Leafs have been upgrade to bigger packs when their batteries were replaced, but that’s always with a pack from a wreaked Leaf. The cost of a new battery pack for a Leaf is more than the car is worth.