I did not start with 12/12. I went directly into 16/12. I am almost 5 weeks since the flip. I defoliated at 3 weeks, but i am now in need of a massive defoliation again due to overcrowding, but i promise you that extending the light hours does work.
It does not restart the clock. Plants just dont know how many hours there are in a day. I myself am doing a 16/12 light cycle. It just works out well. So the point is that (on my light cycle), the plant will get 100 extra hours of light than doing 12/12, over an 8 week period.