I can't recall offhand, but the more I think about it, I'm pretty sure I've seen threads before where people regularly increased their pot size even during flower.
The only thing in your case that I'd be concerned about is that if you're 2 wks into 12/12, then you're probably still in the middle of the stretch, and I'd just be concerned about possibly stressing or stunting them during that phase. I'd like to hear a few more opinions, but I'd even consider waiting 4 or 5 days, or whenever the stretch is done, and then do the transplant.
But either way, if you think they'll get rootbound and will affect the health of the plant or your yield, then I'd transplant them into the 5 gal.