If an NL player is on your roster and he gets traded to an AL team he remains on your roster until you drop him. If you keep him in your lineup each day it will actually look like you're getting his stats while he's on the AL team, but you aren't, and no additional stats will accumulate for your team from that player. If you drop him the transcation will show the AL team next to the player's name but he won't be there to add by anyone else in your league (usually the next day). An NL FA who gets traded to an AL team usually disappears from the players list the next day. Also, if you look at your "Team Log" page after the person is traded it will show the AL team, but don't worry you do get to keep all of the stats they accumulated while they were in your lineup as an NL player!