I'm in the exact same boat here. You have 30 days to return/exchange it whether opened or not. From what I've seen it looks like I would need to wait through the line like normal. Although, if you contact Apple you might be able to send your back and get a new one. However, if you do that it will likely take a week or two to process due to the current backorder plus the shipping both ways. Best bet looks to be to wait a couple weeks and try then. Who knows by then you may decide 16gb was plenty, or that you actually want 64gb.
Contacting your store in the morning will likely be your best bet. Who knows... they may be nice and process your exchange before the sales as you technically already have the device. Might be semi-likely if your device is still unopened too since they can just give to to the next customer...