Hey,
Let's say Player A has 160CA and 180PA
His all current attributes and all current positions match exactly his 160CA, everything in balance
You make changes by using FMRTE and add few Natural positions with “20” rating but don’t change anything else.
The effect of your action would be declining of his attributes after some time when the game recalculates his attributes and his positions to match his 160CA
New positions costs some CA(let's say 3CA) and this CA will be taken from his current 160CA - 3CA so in that case there'll left only 157CA for his attributes so they slightly decline but his overall CA will stay the same 160CA
Please notice that adding new position won’t affect his development rate it's a different matter.
Thanks for help and knowledge. Well it can affect his development because if his CA will drop little bit then automatically his PA will be looking worse than it could? Also i am curious what is the most important thing for developing young talents.