That's hard to say. Will your good player take one trained slot ? Is one good player good enough to stay in D.2 ? What players does he has around him ?
Usually speaking it is much more interesting to play at a higher level, as the income in D.3 is very poor. Unless you're taking a trained slot with your star player.
Another solution would be to sell your star player, buy 1 or 2 decent chaps to hold in D.2 and still train 3 youngsters, because you'll get a huge amount of money for your player at this age.