Captains - Essentially you can only make ONE change, once a player is locked.
You CAN change captains on the same day once the first game has finished.
For example I could go for Kane at 1pm Monday, then switch to Dumfries 4pm Monday.
But that would be my only switch for MD1.
Subs are similar - you can substitute a player on the same day within a matchday as long as they haven't played yet.