I have had to do this with purchased clones already well into flower. It takes several weeks, especially if they do not have good roots to start with. My three kush x trainwrecks are now bushy little freaks. Lots of strange growth to start, then normal leaves. I have renamed the strain to "Krazy Train".
One thing that sucks, they show the alternating growth of a mature plant rather than 2 shouts at each node. I will (try) take clones then throw the girls in to flower when there is room.