5th week might be to early but I have heard that you can switch the last 2 weeks.
Rasta Jeff from Ire genetics swear by 18/6 the last 2 weeks and that it shorten the flower period and make better flower.
Are your cucumbers in movable pots? If so you can put them near a window during daylight, then have them in with the bud plants when the 12/12 light pops on. If you do 18/6 and start to reveg your bud, all your work will be ruined, if you try then try to switch back your plants can seed itself, make non usable smoke, or even die