Question aboht old gladiators

A question here: I still have my first galdiators, which are now too weak to fight the arenas. They are in rank 3, level 6 or 7 (all 4 or 5 stars). In the other hand, i'm getting newer rank 4 gladiators, better than the olders. What should i do? Evolve the olders or sell them and buy newer rank 4?