With the new training system each position has some particular abilities to train (the ones which are highlighted) , so when you learn a new position to your player, he will have some others skills to train. Often he won't be good in these new skills so I have listed the skills you gain when you learn a new position in order to see the difficulty of learning every position.
I don't know if I'm clear but here the list :

DL/DR --> DML/DMR : Passing + Strenght
DL/DR --> DC : Heading + Strenght

DC --> DL/DR : Crossing + Speed
DC --> DMC : Passing + Creativity

DML/DMR --> DL/DR : Speed + Fitness
DML/DMR --> DMC : Heading + Fitness + Creativity
DML/DMR --> ML/MR : Dribbling + Speed + Creativity

DMC --> DC : Nothing
DMC --> DML/DMR : Crossing
DMC --> MC : Dribbling + Shooting + Speed

ML/MR --> DML/DMR : Tackling + Marking + Bravery + Aggression
ML/MR --> MC : Tackling + Marking + Bravery + Shooting
ML/MR --> AML/AMR : Shooting + Finishing

MC --> DMC : Heading + Strenght + Aggression
MC --> ML/MR : Crossing
MC --> AMC : Heading + Finishing

AML/AMR --> ML/MR : Positioning
AML/AMR --> AMC : Heading
AML/AMR --> ST : Heading + Positioning

AMC --> MC : Tackling + Marking + Positioning + Bravery
AMC --> AML/AMR : Crossing
AMC --> ST : Positioning

ST --> AML/AMR : Crossing
ST --> AMC : Nothing

So we can see that some learning are more fit than other (ST --> AMC, DMC --> DC where your player won't have a new skill to train) and learning in a way is less difficult than the other ( for example learning your MR to be a MC will be more difficult than learning your MC to be a MR)
I could also continue with combinaison of position but I know that learning a position isn't really use because of its cost so I will wait until answers