You can flip them. But it is usually best to do it all at once. If it is a player you know you want, dump a bunch effort on him all at once and get rid of the SIMs. Unlike human players, SIMs won't come back after they are knocked off a guy. So just get rid of them early and be done with it.
Just know that at D3 you get $3,000 per scholarship. If you are battling for a SIM for a player, it is going to take all $3,000 if not more to get the SIM off of him. So you can't go fighting SIMs for all of your players. You have to go out and find some undecideds which will let you save some money to battle for recruits you really want.