Doing 12/12 When to switch from MH to HPS?

Doing a 12/12 grow from seed with 400 HID light, I have the MH bulb in it now and the plants are over a foot. When should I switch to the HPS bulb? Its an AK47 using the drip method approximately 6 weeks old.

I'm doing 12/12 because I screwed up and forgot to reset my timer and too late to switch back to 18/6 so I figured I would give 12/12 a shot.

Any input greatly appreciated.

If you go 12/12 from the beginning why not to go with HPS from beginning?
I mean 12/12 is for flowering so why not to provide it with proper light for flowering?

Yes 12/12 is the hours for flowering but the plant still goes through a vegetative state, I'll just wait till I see some buds start to form them switch to the HPS light.

use the hps light. it will work out better than the MH