There's a way you can downgrade right at this moment without another transfer, as long as you have a pre-transfer backup on your hacked 3DS.
Download the DSi game on freeshop, install the save, then go to system settings and back it up on the SD card. Buy the same game on the unhacked 3DS, give it the hacked 3DS's SD card, and import the backup you made.
You will then have he hacked game on the vanilla 3DS.