Can you "train" a cockerel to replace a mean Roo??

[Oven Ready]

I sure hope it doesn't come to that

It IS possible that they won't fight and your young boy will stay subordinate, but it's in the nature of male chickens (and not just chickens either) to become the one who passes on their genes.

If it were me, I'd leave them alone and not mess about with what they do naturally, if they have a proper fight (and they both survive it) you can then decide which one survives as rooster.
It's possible to have an ex-rooster hanging about, we've got one but it's quite a sad sight sort of like an old man in an old-folks home just waiting to die.​

 

[PoultryPedia]

I have found that a roo that is handled a LOT as a chick may tend to get cheeky and consider you both "one of the guys" and "one of the girls" when he gets older--He'll enjoy cuddling, offer you food, AND threaten you with attack. He is likely to require some extra work if he needs gentling when he is older.

A roo who was not handled much as a chick is easier to cure of aggression.

But in either scenario, it seems the majority of roos can be cured of excessive aggression.