Yea, you can switch now but it might pay off to wait until they grow another 6 inches or so if you have the space and the patience. Your plant could mostly indica (a lot of indoor seeds are) in which case it will grow to maybe two feet once it starts to flower. You might also have a crazy sativa, in which case you'll end up with a 6 foot tree if you flower now.
Ideally, you don't want them to grow over 3 feet with a 400w because the bottom branches won't get any light if they're any taller. Then again, if you have the headspace and trim off the crap at the bottom, it doesn't really matter. And if you only have two plants under that one lamp, you'll want to give em some good veg time if you want to pull a nice harvest.
If I were you, I would give it another week or two, take 6 clones from each plant and then switch to 12/12. If they end up being females it will be worth the wait and if they're males, you'll at least have learned something....