"What do you mean with sell the old players before buying new ones"
It shouldn't be the same, "keep the key players u have" and, go to the market for replacements, keep both of them (old and new players) and see both in the same shape.
When I talk about "contribution roles", I mean, that the game do a limitation of key players intentionally, or for ex. there's only one good player for FKs (with differance).
So;
-There should be a differance, if you, 1st sell the actual players and then you go to the market for replacements or,
-You keep "your old glories" and you go to the market for same position players,
Cause the game, assigns the contribution that each new player will give you, in relation with the team you actually have.
If you have a scorer of 40 goals, you can't have 3 for that reason, if you want a new player to score 40 goals this "role, as main scorer" should be free, or change of player.