I think you can substitute all you want and captain anyone then change change, until the end of GW
but your former captain will lose his points
Example: Your captain Lukaku made 6 pts and you think another guy would make more points and you give him captain, and that dude say Ronaldo scores only 2 cap points in his game and it is the end of match day
You'll lose Lukaku's captain pts and gain Ronaldo's 2 captain points and that is valid for the end result.