Out of necessity, I had to finish vegging a couple plants under my HPS before I switched over to 12/12. Its something I will never do again (I'll make sure I time it so my plants come out of flowering just as other ones are ready to go in) because it made them much lankier than I've ever had, even with the fans blowing on them. My yield doesn't seem to be decreased so far, but the plant height is about a foot taller than I normally grow.