Fuck. I wish someone had picked up on this immediately. You can't system transfer from a new3DS to an old3DS, you'll need another new3DS.
On the plus side, you don't have to worry about whether or not you can send back the one you purchased, since you won't be using it at all now.
The used to be a "total" transfer limit, this has been lifted. I haven't heard about the "3 times per year" limit
@Hyoretsu mentions before, so I can't speak to that.
Recent developments seem to indicate you'll be able to use "almost any" DSi to downgrade. They can't remove "almost every" DSi title can they?