On the fifth tab in the game plan, on the right side of the fourth tactic is a menu called "tactical assistance". In this menu you have options for changing how the system provides tactical assistance during a match. It contains options like:
- offside trap - allow the system to activate the offside trap
-change formation - allows the system to change formation during a match
- substitutions - indicate to the system at what point do you want it to make substitutions for you, early, very early, flexible, late, very late or off
- attack/defence level - normal, defensive, very defensive, attacking, very attacking and off (if you want to manually change the levels during the matches)
below it are the options to assign sub routine to the d-pad - offside trap, CB overlap or switch sides
and below this is an option to select sides bases on specific criteria - form, ability, unedited.
You just need to change the "change formation " to off. There I detailed the full menu for you...