Active "locked in" players are moveable within your active lineup even after their game has started.
Say I have a 2B and SS eligible position player who's game starts at 11:05am. He happens to be in the 2B position on my active roster. My SS who's game is at 7:05pm sits, but obviously I won't know that until 5pm or so, when my 2B has already played his game. I should be able to move my 2B to SS and bench my inactive SS, allowing me to fill 2B with other eligible players on my roster.