If you want to go from a 4-4-2 to a 3-4-3 for instance then just sub one of your bench strikers for one of your starting defenders. It's as easy as that.
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Don't forget to set your captains, guys. The captain gets double points for the gmeweek. And the vice captain will become the captain if the captain doesnt play.
Also, the order of your bench will be the order of the subs the game will make if one of your starters doesn't play. So make sure you set them the way you want.
It worked for me on like the fifth time of trying, I copied and pasted it each time from your first post...but it seems like I was the only one to have a problem, maybe my team is just too good for your league