switching bulbs/nutes from veg to flower

I have been growing with a MH 18/6 and I am ready to start flowering. When should I change bulbs? A. when I change the timer to 12/12 or B. when I start to see the first signs of pistils. If it is B will the plant stretch less? The same question but with the nutes. Change nutes when I change the timer or see the pistils?

You can start flower with the mh to reduce stretch the first couple weeks.Change bulbs when you want to.Change nutes when you start flowering.