Is that plant in the background getting as much light as the ones in front?
Here's my experience with a fast switch to flowering: It took the plants another 3 weeks to really start showing pistils. I switched them after 3 weeks, and they were all growing very vigorously or I wouldn't have. But it seemed to take significantly longer to see pre-flowers on them. HOWEVER, all that being said, it did keep them shorter, which was what I wanted for my small grow box.