Originally Posted by BaseballMan
Simple and none of that is problem. Just change all NA, UA, PL team abbreviations to NL. Change AA to AL teams. Do the same for batting, pitching and fielding.
IIRC, only the team file needs to be altered. The batting, pitching, and fielding files only use the team ID not the league. The team files need to be altered because there to match the structure of the league.
Ideally, 19th century play would be able to fold and create new leagues automatically but that would be a major recode of stuff I that has been there forever.