If youo're not having success with the ammending, transplanting is an option. But only you know if you are able to pull this off. Kind of an advanced thing, but I know the frustration of fighting soil ph. Usually I'm a "fight it out and learn from it" supporter, but bad soil ph is a problem worth correcting if possible.

Existing soil in a transplanted rootball will still be a tad stressful, but any time's the right time for good soil and proper ph. You'll have to gauge the risk/reward ratio and make your own call.
Still in veg now, right...? If so, I'd have no problem with gently 'massaging' a bit of the soil from existing semi-moist rootball. Perhaps the morning before the next day's watering, for instance. A dry rootball can snap-off large chunks of roots, and a wet rootball can be a bitch to keep together, and it's heavy. Don't be too aggressive and start tearing, folding, spindling or mutilating the roots, and don't snap the branches off. If quick, you can gauge ammount of soil you'll need in the bottom of the new pot (by using that rootball in your hand as a gauge) and fill the bottom of the pot to desired level. Don't let the roots sit in direct light and don't let them dry out. Once you get the right ammount of soil in the pot, (sticking with 3 gallon, or upsizing...?) transplant as normal. New soil all around it and some on top of the existing rootball. Not too much, just enough to give a bit of assistance to the old rootball. Water it in, fill voids and water once more. Good to go.
Let heal for a week or two before assaulting her with chemicals, additives, schedule changes...Let soil dry-out between waterings. If going with FFOF, likely won't need the nutrients till actively flowering 'em.
Also, you might want to try stuff like this on one plant at a time, and have tools ready before you operate. :thumbsup:

p.s. I don'u use Fox Farms soil, but I do use their nutrient trio. But I hear the FFOF is some good stuff.