I have the same problem also, I have a 81 overall SG and a 75 overall SG, I wanted to start the 81 and I did reorder the roster, but still every game the 75 SG starts, what I noticed is the 75 SG is better in FG,3 PT and FT than the 81 SG.
Try this one: Play a game,then after the Match Up window comes the Starting Lineup window right? Choose who you want to start then press ok and start the game. During the jumpball phase, quit the game. Simulate games and see that the starting lineup you arranged will be the lineup for all the games in the season unless a player or two gets injured(In that case, do the sequence stated above again)
Have fun
