"Player's natural playing role is Quarterback
He can also play adequately as Linebacker
If really needed, he can also step in as Offensive Lineman, Defensive Lineman, Safety and Kick returner"

That is one of my players. His positional rating on his page says 2 stars.

Ideally, I want this guy to be a CB. But for now, I need to play him at FS because I don't have anyone else for FS.

If I play him at CB, will all the positional ratings for all those above positions drop? Or will they stay the same and just the rating for CB would go up?

And could I then just play this guy at CB for league matches and FS in friendlies and get both positional ratings up?

Also.... are any natural positions related to each other? I mean, would a FS be more adept at playing SS or a DT be more adept right away playing DE because of similar skillsets (because of the "natural position" ).

Honestly natural position probably has to degrade slightly but we really can't tell without assistance from another squad making weekly scouting reports on your player. I make the assumption that natural position degrades based on the fact that if it doesn't then once a player hits five stars he is unchangable from that position otherwise.

well it doesn't have to degrade rapidly