Simply select “Show Design Templates” (Window menu), click on the cart icon in the toolbar and enter the transaction ID to re-download your design(s).
You still can use Links with “http”. Linking to your eBay store or categories using http shouldn’t cause any trouble. You just shouldn’t use HTTP content like “http” image URLs any longer.
However, to answer your question: In theory you can change your link from http to https as long as the server supports it! So you better test it first!