It's probably a silly question, but I'm trying to talk myself into getting into the game and I don't want to lay down the cash for the whole package right now. And I don't trust Nintendo with things that seem obvious. Any reason one couldn't play on his old 3DS with the cart and continue their game on a n3DS with the improvements later?

As far as I'm aware there shouldn't be any issues with doing that, hell I've been playing it for 30 hours on the original 3DS and don't plan on getting a n3DS anytime soon. It plays completely fine.

For me the increase in battery life alone made going from the launch 3DS to the N3DS worth it.

They do offer a download on the eShop that lets you do cart save transfers, so I can't see there being any issue.

I moved all my stuff from an OG 3DS to a New 3DS and had no issues with any of my games, cart or download.

I did this with Monster Hunter 4U, actually. I played like 10 hours on my 3DSXL before I decided the c-stick and better textures was worth it, transferred with no problems. Just remember that you'll have to do a partial wireless transfer and then do an SD card transfer w/ a computer for the quickest and easiest way. Also, since the N3DSXL comes with a 4 gb microsd, so you may need to buy a bigger one. I got a 32gb microsd for like $15.Just use this video on how to do the transfer properly, it's kind of arcane.

I did exactly this, played about 10 hours on my original, small 3DS and then transferred over to a New 3DS XL. No issues transferring the game data over, but you do have to follow the arcane system transfer process which took me about an hour to do even though I did the fastest method of using a PC to copy data from my regular size SD card to a microSD. Even though I originally just wanted to play on a larger screen, I have to say that the C-stick nub is a revelation. It makes control feel so much smoother and I can have quick access to a few more items on my bottom screen now that the virtual D-pad isn't taking up that space.

Fastest way is to grab yourself a decent sized microSD card (with SD adapter) then use a PC to transfer content from your old SD card to the new micro.

Use that microSD in your 3ds for now and when/if you pick up the new 3ds, the transfer process is a lot faster. Transfer settings wirelessly from old 3ds to new, then just pop the microSD out of the adapter and plug it into the new 3ds.

I did this last week, the only slow part is transferring the settings from old 3ds to new.