Okay, so this is gonna be somewhat long, but I'll try to make it as short and clear as possible and I'd appreciate A LOT if someone can help me! 
All right, here's the thing:
I have a 3DS and it has a 16 gig SDHC card that (of course) has everything (my downloaded games, demos, videos, photos, music and more) on it. The day after tomorrow I'm getting the A Link between worlds zelda 3DS XL , and after researching, here's the question 1:I figured that if I want to transfer everything from the old 3ds to the 3DSxl, I'll have to do the system transfer from settings, but how will I be able to tranfer everything to my 3ds xl if the sdhc card from my old 3ds is in it during the transfer? because I think everything won't fit in the included 4 gig sd card in the 3ds xl while transfering. And I can't just put the 16 sd card into the 3ds xl without doing anything, (it won't accept it)
the question 2 is: Let's say I somehow managed to get question 1 solved and get everything to my 3ds xl and now the old 3ds doesn't have anyithing anymore. I want to give this old 3ds to my little brother, but if I have already transfered everything to my 3ds xl, do I put a new empty and blank sd card to the old 3ds? (btw I know all the built in apps like the music and 3ds camera will be there, because they aren't in the sd card), I'm doubtful either to do that, OR putting the 4 gig card that came included with the 3ds xl into the old 3ds, because that one will have all the info and stuff that comes already installed.
Sorry for the legthy question, it's just that I just thought about this and freaked out!
Thanks a lot for minding in reading this question, I would really appreciate a good answer 