send the fresh pair in the fresh box back for the refund.
the box should be exactly the same with the same barcode and UPC.
Their return policy for unworn shoes is pretty good. They don't have a specific timeframe listed on the site but I've asked a lottt of chat reps and they've said 2 years from manufacturing date for returning unworn shoes in original packaging. You can chat with them if you want to verify it, once one rep tells you that then they will need to stay true to it.
As long as you're ordering the same size, you can return the new shoes under the older order. It's basically a long term price adjustment you just have to ship them back