Customers don't switch to a new SKU because the packaging changed. They switch when the upside of trying it is meaningfully larger than the risk of being disappointed.
A money-back guarantee tied to satisfaction, or an instant-win layered onto first purchase, converts 'why would I?' into 'why wouldn't I?' Both are underwriteable at a fixed cost the brand can build into launch P&L.
The most useful post-launch metric isn't sell-in — it's repeat rate among promotion-acquired buyers. That tells you whether the mechanic pulled the right customers or simply the cheapest ones.