Hi

Not sure about new players decreasing chance to score.. It seems totally random for me..
Have great new players as well as very bad ones on the first few matches.

Over 18 seasons with my main team, I have changed formations a few times.. I usually do not change my formation to counter the other team, but rather, choose a mentality, whether it is defensive or attacking, and then choose the formation accordingly. I fount it is fine to change the formation to whichever formation you want as long as you know your players well.

If you have a formation and philosophy that you like, it is easier to replace the player rather than changing the whole thing.

If you are successful with that, I think you do not need to change much, but if you keep losing, then it is good to change the formation and strategy of your team.

My believe is "Why change it is working effectively?"