You can change them up until the deadline... there is a rule that says NEXT DAY, but, ive had no problem changing them for players by the deadline.
BUT, if you played the Thursday game and its already started, NO CAN DO!
Why I advise players to never take the Thursday games unless you are 100% sure its a winner, cause shit like this happens. Wont know about some freak injury till during or after the Thursday game... and you had Texans -13, and stuck with it now
Now, people that used a couple other services would be unable to change their plays before game-time, or if they are, they would charge them a fee.
IF you enter, be sure to ask your proxy LOTS of questions like this, and what their rules are about changing plays or if you send them a little late, etc... all good things to know and ask beforehand.