Looks like you're getting good advice so far.

You need to gently slide the plant(s) out of the pots and have a gander at those roots. That's one good thing about those tapered square pots; they make that easy. Use a "frosting knife" to loosen things up if necessary.

If they're rootbound, it would explain your apparent N deficiency symptoms. Adding more N won't help if you've got a lockout. (A flush followed by a light feeding usually temporarily improves lockout symptoms, so you could try that with a plant or two as a diagnostic tool...but you'll still have to deal with the cause.)

If they look rootbound, they'll need potting up, but IMO only to the next larger available size. Repotting won't stress them, they'll love it if they need it. But most strains I've worked with lately haven't generated much new root growth after the 12/12 flip. That's why I'd advise a pot size increase of only 1 increment. Plants at this growth stage may not drink a 5 Gal container dry enough between waterings, and sloppy soil will give you a similar-looking set of problems.
DreadedHermie Reviewed by DreadedHermie on . Fan Leaves yellowing and browning 3 wks into flowering What is your experience level? (first timer, novice, experienced...)1st timer Your Equipment: .1) Type and wattage of lights. (MH, HPS, CFL's, tube fluorescents, LED's)600w HPS .2) Distance from tops? 1-2.5 ft .3) Reflector type? (cool tube set-up, bat wing, enclosed reflector, bare bulb...)enclosed air cooled reflectors .4) Is there a consistent fresh air supply?Yes .5) Do you have an exhaust fan and a circulation fan?Yes .6) What are the bulb wattages, kelvin ratings, and Rating: 5