YukonA4

hi there I know this has gone threw a few times but im needing help on this. I have a 1996 A4 with the 30v swap, blew the motor long store haha. But i got a new motor sent to me and well its a 12v and im getting hit with over 1200 in shipping to get it there and a new one back. Is there anything I would need to know about swaping the 30v back into the 12v. I know this sound so dumb but the 12v motor only has 20k on it so im working with what I have. Would i need to swap out the ecu and harness ? i know the mounts and tranny are fine **** asking a few little things before I go in and do the swap.