You can, yes, but it’ll take quite a while! You need to do a system transfer to your New 3DS XL from your New 3DS. You can then redownload Majora’s Mask on the XL as the system transfer will delete it (redownloading it from the eShop will be free however, so don’t worry). Then you have to wait several days (14, I think, before Nintendo allow another system transfer), after which you can system transfer everything back onto your Ambassador New 3DS.