I have mixed results with training on new positions, but the thing is: if you already have the player and he's worth very little, selling him won't do much good either.
So with youth players I already have, I tend to try it, and if the player starts to whine, I'll just try to sell him anyway.
But in that case there's no real drawback anyway, as the alternative would be to sell regardless. So I think it's worth a try in those cases.
With main squad players, things are very different. I don't have time to retrain those, and I can't allow players to start a riot.
Also, main squad players tend to be easier to sell, even if you don't use GFRay's trick with value at zero and negotiate using the player trading thing.
So for main squad players I tend to just sell and buy players who are actually happy to play in the position I need them to play.