I don't think transplanting a 12/12 from seed plant during flower would be worth it because their life is so much shorter and they have to put their effort into flowering (and not root mass growth) since they have so little time left to live. If they are starting at 12/12 from seed they might only veg the first 2-3 weeks (roots grow more in veg than flower), when they switch to flowering less effort goes under ground and more effort goes above ground. Plant them in the bigger pots in the first place and things will go MUCH better than if you transplant.
Now if we were talking about transplanting plants that had been vegging on 18/6 (lets just say that) and have since switched to 12/12 and you want to transplant them go right ahead as long as its within the first month of 12/12 (first couple weeks most ideal time to transplant in flowering). Most advise to transplant just before switching to 12/12 which is what I would do.
Bottom line: 12/12 from seed should be planted into the largest containers possible to maximize growth rates, reduce stress, and increase yields.
Ideally one shouldn't ever go 12/12 from seed if they don't have to, 12/12 from clones would be much better (but still not as good as some veg time before 12/12).
