A few days on 18/6 isn't going to buy either you OR your plants very much - but, as Ezzjaybruh said, expect a LOT of stretch from them when you flip no matter whether you go 18/6 for a few days or immediately go to 12/12... I hope you have room to move your light up as they do their stretch! If not, you might want to think about SCROGging her (although it's pretty late to do that, it might have some benefit) or prepare to do a lot of LST to keep them from kissing the lights!
Good luck!