yes...it is possible but it will reduce the yield of the plant. lets say the normal plant gave you a ounce, if you switch it will give you an 1/8th. if i were you i would leave it alone. just keep it in that cycle and watch the magic happen. you clone will double in size while in flower stage.
next time, leave the clone under 18 on 6 off and you can veg as long s you want. but dont switch it back...all you do is ruin what you started.