What if you veg with 600 and flower with like 1-2 hundred watts

Will thus ruin the grow

Probably not, but you'll severely reduce your yield potential.

Why would you want to?

^^^^^^^^^^^^^^^^^^^^^

If anything you want to do it the other way. low watts in veg and more in flower.

You can usually get away with topping and training out an overgrown plant to tame it as opposed to lowering wattage. Then make more clones with the cuttings....