Personally, I'd go ahead and transplant into a larger pot now, and keep 'em on 12/12. This will give you a better picture of what the plants are likely to do in flower so next time you can plan from there, keeping headroom restrictions in mind. Switching back-n-forth gains you limited knowledge, and can stress 'em out.

As long as you've got alternating internodes, they are old enough to start flowering. You just lose some of the bulk and height that you'd get with a longer veg.