When to switch lights?

[Surfer Joe]

When going from veg to flower, we switch to 12/12 light, but keep giving veg nutes until we see the buds starting then switch to bloom nutes.
We also change lights from MH to HPS.
my question is-
when do you change the MH to HPS?
as soon as you switch to 12/12?
Or when you see buds forming and switch to bloom nutes?

 

[WeedHopper]

I switch to bloom when i flip.

 

[pute]

Yup

 

[ROSTERMAN]

Yeppers