Animal Crossing New Leaf – How to Retain Save Data with a New Copy

You can not have multiple save files of individual towns on the same 3DS system.

Every time you save your game it generates a new, random "Save ID" and stores that ID in both the SD/Savegame and the 3DS's system memory. Trying to load a save with mismatched IDs will result in being forced to wipe the save.

This prevents two things:

1. Having two towns with one copy of the game by using two SD cards.
2. Reverting to an older backed up save to undo changes you aren't happy with.

http://gbatemp.net/threads/question-about-id-checks-with-3ds-and-new-leaf-save-files.349216/

I have personally tested this (having purchased a 3DS and transferring to 3DSXL- my game was cleared since the save data and the system ID did not match). Prior to this, we attempted backing up the SD data, erasing it from the SD and attempted a new save file on the same digital copy. This also resulted in the ID conflict. I have contacted Nintendo on this matter causing an inability to back up data. I am yet to hear a followup reply.

In the case of mismatched IDs, the game will load with a "corrupted data" error. This forces the system to erase any save data associated with the digital download- and any backed up save file will no longer work for AC:NL.

To use this cheat/hack, you need to change your 3DS time (in system settings) not the AC:NF time that Isabelle asks if you want to change. It is best to travel as far as you can into the future (the year 2050 I think). Then you will get a letter from the post office saying that you have gained a certain number of bells and they can be taken out at the post office bell point. Something to remember though, you will get lots and lots of weeds throughout your town if you do this multiple times.

NOTE: You only get interest after a month, so a couple of days difference will do nothing.