when to switch to HPS?

What is popular opinion on when to switch to HPS from MH...

first day of 12/12?

After a week? Two?

The thought being that if the plant sees a switch of light hours AND spectrum it might freak out and think is late...going male or hermie.

Keeping the plants under mostly blue light (MH) for the first 2-3 weeks of flowering can significantly reduce stretch.