Tactics and formation problem, striker is defender for example

Hey, 

i startet a carrer mode with Boca. 

So i created this new tactics. 

An Offense and defense one. 

When i take the shortcout during a game for example to offense. Cavani is a defender, my leftback is a striker. 

But i cant change this from the beginning, i create a tactic and then nothing. I can only chance the players with my standard tactics. 

Or is there another way to change this before the match?

Kind regards