Yellowing leaves during flowering

I have six plants that are just getting into the flowering stage. The leaves of my bigger plants have started to yellow and droop when I went to a 12/12 light cycle, while my three smaller plants show little or no yellowing and some drooping. A friend of mine said that a plant does redirect energy during flowering, perhaps this was a normal reaction?

It is 100% alright for leaves to turn yellow during flowering. Your friend is right.

Awesome CCK, I appreciate the feedback and the piece of mind. Thanks!