Actually, you can do this in any type of OOTPB game, I believe, not just historical mode. You would edit the league structure, as the manual describes
here. Click the Import Hist. Team button and I am hoping that 2013 teams would be available if you copied over the database from the OOTP 13 game to the OOTP 12 folder. And that's assuming I am reading you correctly that you have OOTP 13 and want to revert to OOTP 12.
Something tells me I am 0-2.
