The final NBA2K12 roster update went live this morning. All injured players are now active. No trades will be updated since they were not part of the 2011-12 season. All off-season transactions will be in NBA2K13 instead.
The roster will be downloaded automatically as long as your PC, Xbox 360 or PlayStation 3 is connected to the Internet and your game is up to date with all the official fixes. To access the new rosters, use the Reset Roster function under the Options menu.