I would leave your lighting the way it is. Your plants won't grow as fast as they would under 18hrs of light, but you don't want to mess around with your light period now- it will only stress out your plants. you can also veg your plants indefinitely if you want, ie. you can veg them as long as you want before flowering and nothing but good (correct me if im wrong) will come of that.

as for switching the light cycle gradually before flowering...well i personally wouldn't do it, but i think it's sort of just a personal choice for the grower. there are probably dozens of debates on this subject on these boards.