When a product has an updated style or slightly changes sizes, it may be helpful to merge variants so that the new product has sales history needed to create a forecast.
In the Replenishment screen using either the Variants or Bundles view, click on Details (the "i" icon) for your new variant.
Click on the tab for Merged Variants. Then you'll search for the old variant.
The Merge Stock check box should be selected if you want the stock of the old variant added to the new variant for the purposes of forecasting. This will not push stock quantity changes to your connected platform.
Both variants will continue to show as replenishable. If an item is discontinued (a new variant is replacing an old one), then you should set the old variant as non-replenishable. For the new variant, Inventory Planner will pull in sales history of the old variant to create a forecast.