Why would you do 12/12 from seed? Flip your lights to 18/6 and let them veg and prosper. However, if they are growing and not turning to flower yet them yes, you can top them. The problem is that when doing 12/12 from seed your plants will flower at the earliest time that they are mature to so topping them might be risky depending on how far along they are and if the roots have hardened for flower.