Some stores like GameStop offer trade-in program which can save your 3DSXL purchase cost by trading in your 3DS.
However, a number of store associates erroneously believe that you can simply swap out SD card to keep your data. Wrong.
You need to perform a system transfer in order to properly retain your Miis / eShop games / Ambassador status... etc. To do that, you need Wi-Fi connection.
If the game store happens to have stable Wi-Fi connection, they may let you perform system transfer in their shop under their surveillance. But most of the time they don't. What to do?
- Ask your store about their return policy, like if you can try a 3DSXL out and return everything in like-new condition within 7 days for a full refund. Most game stores should allow that as long as you keep the original packaging and receipt.
- Buy XL at full price, go home and perform system transfer. (Shall take 30 mins)
- Return the XL but tell the store associate you want to trade in your 3DS and buy that same XL back immediately. The associate should be able to do a return, then help you repurchase it while applying your 3DS trade value.
- Enjoy your XL with your data all intact!
PS: The reason it's best to return first is because some game stores (e.g. GameStop) cannot retroactively apply 3DS trade-in credit. This has to happen in conjuction with the XL purchase, so a "technical"return is needed.
(Any comments or questions are welcome)