This has to do with the display of your design if you choose to display your products with the product display: https://www.spreadshirt.co.uk/blog/2016/11/03/shop-tip-product-or-design-view/

Of course your products will still be available after the migration. So you don't have anything to worry about :-)
If your shop has had at least one sale since 1.1.2016 ALL your products will be migrated :-)