I've been testing with my own 2 DS' and there seems to be no issue with simple changes like the color of a koopa or adding new tiles, but changing the length of the stages seems to be a bit weird. I am trying to do this in a way that works with download play so I can surprise a friend with crazy levels next time we meet up, so I am wondering exactly what my limits are in how I can mod it and still have it work for them.

An interesting limitation. This got me curious and I tried doing 16 chunks, and to my surprise it works pretty well. Of course the game isn't too happy about it and lags a every once in a while but it worked and it was pretty funny so I'd call it a W. I wonder how 32 works...